Space Science Sampler 2/NASASpaceScienceSamplerV2.cdr/planet/mars/surface/m15west.txt

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                    DEPARTMENT OF THE INTERIOR

                      U.S. GEOLOGICAL SURVEY

                     Digitized version of the:
      GEOLOGIC MAP OF THE WESTERN EQUATORIAL REGION OF MARS

             By David H. Scott and Kenneth L. Tanaka

                             Based on:
       U.S. Geological Survey Miscellaneous Investigations
                        Series Map I-1802-A

 Prepared for the National Aeronautics and Space Administration

                              1986

                          AUTHOR'S NOTE


AUTHOR'S NOTE

This  optical-disk  version of the map of the western  equatorial
region  of  Mars portrays the geologic  units  only,  which  were
digitized  by the U.S.  Geological Survey.  It does not  indicate
surface  coordinates,   structures,   topographic  features,  and
correlation of map units that are shown on the published version.
Map coordinates are given in degrees;  longitudes are west of the
prime  meridian.  This  version  of  the map and  text  were  not
reviewed for conformity to U.S.G.S.  standands;  this work is not
an official publication of the U.S.G.S.

INTRODUCTION

This  map  of the western equatorial region  of  Mars  supersedes
previous  geologic maps based on Mariner 9 data.  It represents a
more  advanced  study  of  the geology  afforded  by  the  higher
resolution,  better quality,  and nearly complete coverage of the
Viking Orbiter images.  The map is the first of three in a series
planned  to cover the entire planet,  portraying  its  lithology,
stratigraphy,   and  structure.  The  text  describes  the  major
tectonic, volcanic, and fluvial episodes that have contributed to
Mars'  evolutionary  history.   The  base  used  for  compilation
reflects   improved   imagery  and  photogrammetry  and   updated
nomenclature.

Geologic units were identified and mapped from  1:2,000,000-scale
photomosaics  and  individual  images,  most of  which  range  in
resolution  from 130 to 300 m/pixel.  The units were assigned  on
the  correlation  chart to the three  time-stratigraphic  systems
previously  formulated (Scott and Carr,  1978) from the Mariner 9
mapping.   Relative  ages  of  the  units  were  established   by
stratigraphic  and  structural  relations  and  by  crater  size-
frequency distributions.

Cumulative  crater  densities of units in the three  systems  are
also   shown  on  the  correlation  chart.   Because  of   crater
degradation,   resurfacing,   and  declining  crater-flux  rates,
relative   ages  of  increasingly  older  surfaces  on  Mars  are
determined  by densities of progressively  larger  craters.  Thus
density  scales  for  craters larger than 2,  5,  and  16  km  in
diameter  were  selected for relative-age correlation within  the
Amazonian, Hesperian, and Noachian Systems, respectively. Overlap
of the scales was empirically determined by calculating ratios of
crater densities at different diameters for units most likely  to
preserve a wide range of crater sizes.

The  Viking map series shares some of the uncertainties  inherent
in  earlier maps of Mars in that many of the primary depositional
characteristics  of  units  have been modified  and  obscured  by
erosion,  deposition,  and tectonism.  In particular, deformation
has  destroyed all morphologic properties of the  materials  that
are  normally  used to infer rock type in the highly faulted  and
fractured  areas  north and south of Tharsis Montes  and  on  the
Tempe  Terra  plateau.  Three units from these areas are  grouped
under  highly deformed terrain materials and mapped  as  basement
complex  (unit  Nb)  and fractured materials (units  Hf,  Nf)  of
different   ages,   but  they  are  considered  to   be   locally
gradational.

The  map  units are broadly subdivided into lowland and  highland
terrain  materials.  The  lowland terrain consists  of  extensive
plains  north  of the highland-lowland boundary scarp  that  lies
mostly in the northern hemisphere.  Much of the lowland region is
covered by small knobs and conical hills.  South of the  boundary
scarp,  plateau  terrain  of higher elevation and greater  relief
extends to the south polar region.

A  rock-stratigraphic  classification  of  both  formations   and
members  is  employed  here for the first time  in  the  geologic
mapping  of Mars.  Its use minimizes the need for adjectival map-
unit descriptions that may be both inadequate and  confusing.  In
the lowland plains as well as in the highlands, many units appear
to be transitional. Some contacts are mapped largely on the basis
of crater-density discontinuities; these "statistical boundaries"
are  dashed.  To avoid obscuring relations among geologic  units,
only  superposed  crater  blankets wider than 100 km  and  partly
buried  impact  craters  larger than about 150  km  in  rim-crest
diameter are mapped. No attempt has been made to classify craters
by relative age according to their degree of degradation, but the
more  significant factor--the stratigraphic position  of  craters
relative  to adjacent terrain--is shown by color.  This  relative
position  is determined by embayment,  overlap,  and  transection
relations.

This  Viking map thus differs from earlier Mariner 9 maps in many
respects,  of  which the following are most  significant:  (1)  a
nearly three-fold increase of mapped rock units over those on the
geologic  map  of Mars based on Mariner images (Scott  and  Carr,
1978);  (2)  the subdivision of lava flows associated with  major
eruptive  episodes  at  Tharsis Montes and other  large  volcanic
centers;  (3)  the  expansion of areas mapped  as  channel-system
deposits,  especially  their greater extension into the  northern
plains;  (4) the recognition of  smooth,  soft-appearing,  easily
eroded planar deposits of possible ash-flow or eolian origin that
cover large areas in the equatorial region of Medusae Fossae; and
(5)   the  addition  of  many  small- to  moderate-size  features
interpreted  as  volcanoes  and  source  vents  in  the  southern
highlands  and on the Tempe Terra plateau.  Also,  several  units
have been assigned to different time-stratigraphic  systems.  For
example,  the  mottled  plains material,  formerly classified  as
Noachian  in  age,  has  been placed in the  upper  part  of  the
Hesperian  System on the basis of crater counts and stratigraphic
position,  and it is now considered to form part of the  Vastitas
Borealis Formation.

Detailed  studies  of the stratigraphy and geology of  Mars  have
increased   dramatically  since  the  Mariner  9  orbiter   first
transmitted  an  encyclopedic pictorial library of  the  surface.
Higher  resolution  data  gathered from the Viking  mission  have
enabled  surface processes to be described in greater detail  and
their interpretations modified or changed.  Much of this increase
in knowledge is attributable to a growing community of  planetary
scientists  and  contributors  in  many disciplines  and  to  the
development  and application of new technologies.  Many  geologic
questions on Mars have yet to be resolved,  but a vast literature
now  exists  that  addresses  such  problems  as  tectonism   and
volcanism in the Tharsis and other regions, development of Valles
Marineris,   the   cause   and   chronology   of   channel-system
development,  and  the origin of the northern lowlands.  Although
the general perception of Martian surface history has not changed
appreciably  since Mariner 9 mapping (Scott and  Carr,  1978),  a
much  more  detailed  synthesis has now  been  achieved  for  the
relative  ages  of  geologic  units,  topographic  features,  and
tectonic events.

A  major  problem  remaining  in  Martian  geology  concerns  the
absolute  time of occurrence of geologic events and the rates  of
surface  processes.  Impact-crater  distributions cannot  yet  be
confidently  calibrated  to  an  absolute  time  scale,  although
attempts  have been made through crater-population comparisons of
Mars and the Moon and through studies of planetesimal populations
(Soderblom and others,  1974;  Neukum and Wise,  1976;  Hartmann,
1978).  The solution of this problem, as well as our knowledge of
the  lithology  of  the geologic  units,  will  be  immeasureably
advanced  when future Mars missions acquire a new  generation  of
remote-sensing and rock-sample data.

PHYSIOGRAPHIC SETTING

A  regional topographic swell centered in the Tharsis Montes  and
Syria  Planum  extends  over  one-third of  the  map  area  (U.S.
Geological  Survey,  1979).  Within this high region are the four
largest and youngest volcanoes on Mars: Olympus Mons, Arsia Mons,
Pavonis Mons,  and Ascraeus Mons.  The latter three  collectively
form  the large,  northeast-trending,  volcanic mountain chain of
the Tharsis Montes.  This volcanic chain lies athwart the  global
highland-lowland  boundary that transects the western  equatorial
region.  The boundary,  where not covered by young lava flows, is
marked  by a gentle,  irregular scarp bordered by clusters of low
knobby hills.  It separates the relatively smooth, flat, sparsely
cratered northern plains from the higher,  rougher,  more densely
cratered plateau terrain of the southern highlands. A vast system
of   canyons,   the  Valles  Marineris,   originates  in   Noctis
Labyrinthus,  just east of the Tharsis-Syria swell,  and  extends
eastward for several thousand kilometers.  Ancient river channels
appear  to  have sources in canyons and chaotic terrain north  of
Valles  Marineris  and  in a large fissure at  the  head  of  the
Mangala Valles. Visible remnants of large circular basins are not
as common on Mars as on the Moon, probably because of more active
erosion  and deeper burial on Mars by eolian and fluvial activity
and  by  widespread volcanism on both the highlands  and  plains.
Argyre  Basin,  the youngest and best preserved impact  basin  on
Mars,  lies  in the western hemisphere.  It is more than 1,000 km
across  and has a broad flat floor (Argyre Planitia)  covered  by
eolian  material  and  volcanic  flows;   the  floor  is  several
kilometers below its encompassing rough mountainous rim.

STRATIGRAPHY

The type areas and basal geologic units originally adopted (Scott
and  Carr,  1978) to define the three Martian  time-stratigraphic
systems (Noachian,  Hesperian, and Amazonian) remain unchanged on
this map.  Many other units shown on the Mariner 9 map,  however,
have  been  reclassified or subdivided and assigned  a  different
position  in  the  correlation chart.  The  chart  is  calibrated
according  to crater densities and reassessment of  stratigraphic
positions.

Noachian System

The  Noachian  System  consists of the oldest rocks on  Mars  and
includes most of the units mapped in the southern highlands.  The
rocks  are  generally densely cratered  and,  in  places,  highly
deformed by faulting. Although the Martian highlands have roughly
twice  the  crater density of the lunar  Nectarian  terrain,  the
shape  of the distribution curve of craters larger than 20 km  in
diameter in rock units formed during Noachian time is similar  to
that of the Nectarian (Tanaka, 1984)--the second oldest period of
the Moon (Wilhelms and others,  1978).  The Nectarian, therefore,
may  be similar in age to the middle of the much longer Noachian.
Because  the Martian highlands are dominantly of middle  Noachian
age,  they  may  be  younger,  on the  average,  than  the  lunar
highlands,  which have substantial proportions of both  Nectarian
and  pre-Nectarian  materials.  The fact that fewer large  impact
basins have been found on Mars than on the Moon may be  explained
by  differences  either in the flux of large impacts between  the
two bodies or in the average age of their highlands.  Models  for
overall size-frequency distributions of craters larger than 10 km
in  diameter on Martian highland surfaces suggest either that the
early population of crater-producing bodies followed a log-normal
distribution law (Woronow,  1977;  Gurnis,  1981),  or that  many
craters  in the 10- to 30-km size range were obliterated (Chapman
and Jones, 1977).

The  basement  complex  (unit  Nb)  is  the  oldest  identifiable
material  in  the  western hemisphere of Mars  (Scott  and  King,
1984).  The unit is highly faulted and cratered and has prominent
relief.  Most  exposures occur in three areas.  (1)  In  Claritas
Fossae   (lat  28  S.,   long  100),   the  basement  complex  is
transitional  with other highly faulted material (unit  Nf)  that
forms a raised block-faulted corridor extending northwest towards
the  Tharsis Montes.  (2) On the north edge of the highland Tempe
Terra plateau,  an outcrop that forms a degraded ridge  (centered
at  about lat 45 N.,  long 84) along the northwest margin of  the
plateau  appears  to have been uplifted and tilted by  high-angle
faulting.  (3)  In the highlands around the Tharsis-Syria  swell,
relatively   small   isolated  peaks  project  well   above   the
surrounding  plains  that are flooded and partly buried  by  lava
flows.  The  basement  complex may also be exposed in  the  lower
walls  of Valles Marineris and in some of the  mountains  ringing
Argyre  Planitia.  The  relatively  high relief  associated  with
basement remnants may be due partly to erosion of a formerly high
regional surface,  but it is also the result of structural uplift
along  normal faults produced by early tectonism and possibly  by
impacts.  The basement complex and older fractured material (unit
Nf)  predate major tectonic episodes and are possibly older  than
the beginning of the Tharsis-Syria swell (Scott and Tanaka, 1980;
Plescia and Saunders,  1982). These units probably consist mostly
of  impact breccia formed during early stages of  high  meteorite
flux, similar to the breccias of the lunar highlands.

The southern highlands consist largely of the seven rock units of
the plateau sequence.  The highlands nearly encircle the Tharsis-
Syria  swell,  extending  from Terra Sirenum in the southwest  to
Aonia and Noachis Terrae in the south to Meridiani,  Xanthe,  and
Tempe  Terrae  in  the  east  and  north.   Their  continuity  is
interrupted  by  channels east of Valles Marineris and  by  Kasei
Valles.  Some  units in the sequence have clearly  distinguishing
morphologic    characteristics,    whereas    others    represent
transitional stages in resurfacing of the highlands by lava flows
and eolian and fluvial processes.

The  hilly unit (unit Nplh) at the base of the  plateau  sequence
forms  a rough terrain of irregular peaks,  ridges,  and  ancient
crater  rims  separated by relatively flat areas.  It is  not  as
intensely faulted as the basement complex,  but the two units are
difficult  to  distinguish in small  isolated  outcrops.  Typical
exposures  of  the  hilly unit occur around Argyre  Planitia  and
along the southwest edge of Daedalia Planum. The prominent relief
of  the unit appears to be due in places to normal  faulting,  as
indicated  by the linearity of steep ridges and  hills.  In  some
areas  the  hilly unit contains small- to moderate-size  features
interpreted to be volcanoes. Around Argyre Planitia, the floor of
an impact basin,  the unit forms features that range in size from
small  rounded  hills to large massifs.  Here  the  unit  closely
resembles  the  Alpes and Montes Rook Formations  encircling  the
Imbrium  and Orientale Basins on the Moon (Wilhelms and McCauley,
1971;  Hodges,  1980),  and  it is interpreted to have a  similar
origin.  The smaller hills are probably ejecta; the larger blocks
appear to be uplifted and tilted crustal rocks.

The  most extensive unit in the Noachian System is  the  cratered
unit  (unit Npl1),  distinguished by a high density of craters in
all  sizes,  particularly  those larger than 10 km  in  diameter.
Craters  are both partly buried and superposed,  and  intercrater
areas  are  rough but without the prominent relief of  the  hilly
unit.  The  occurrence  of lava-flow fronts and  a  profusion  of
impact  craters  throughout the unit suggest that it consists  of
volcanic  material interbedded with impact breccia.  Where  these
materials have been highly dissected by small channels,  they are
mapped as the dissected unit (unit Npld). The small channels have
patterns  resembling those of terrestrial streams and may be  due
to  runoff from ground-water seepage (Pieri,  1980,  p.  148)  or
rainfall (Masursky and others, 1977). Some of the larger and more
pronounced channels are mapped individually.  Where the  cratered
materials are etched into irregular grooves and hollows, they are
mapped  as the etched unit (unit Nple),  which exhibits  features
similar  to those on Earth that are sculptured by wind,  such  as
yardangs, deflation pits, and depressions (McCauley, 1973).

The ridged unit (unit Nplr) also occurs throughout the highlands.
It  is  traversed by rough,  prominent,  sublinear  to  irregular
ridges,  which  differ in size and form both from large ridges in
the  older  hilly  unit and from wrinkle ridges  in  the  younger
ridged plains material (unit Hr).  Generally the Noachian  ridges
are  less ordered in continuity and spacing and have more  relief
than  the  Hesperian ridges,  but they follow the  same  regional
trends,  forming  a great arc around the south end of the Tharsis
swell.  In places the ridges have steep flanks that may be  fault
scarps.

Many areas within the highlands have been smoothed and subdued by
a  mantling  material that is interpreted to be of  volcanic  and
eolian origin.  Crater rims,  ridges,  and hills are recognizable
beneath the mantle,  but smaller irregularities,  which elsewhere
contribute  to  the  coarse texture  of  intercrater  areas,  are
buried. The mantle and underlying material are mapped together as
the  subdued  cratered unit (unit Npl2) of the plateau  sequence.
Other  highland  areas  are  more  thickly  covered  by   younger
materials that form the smoother,  relatively flatter surfaces of
the  smooth unit (unit Hpl3);  its Hesperian age is indicated  by
crater counts.

Rock  units  within  the highlands and lowlands  that  cannot  be
either  recognized on the basis of morphologic characteristics or
placed precisely in a stratigraphic position within the Hesperian
or Noachian Systems are grouped as undivided material (unit HNu).
This  unit  is  mapped in the walls of Valles  Marineris  and  in
chasmata and deep channels.  It also forms clusters of knobby and
rounded  hills  north  of the  highland-lowland  boundary  scarp.
(Southward from this scarp,  these hills are progressively larger
and  less degraded and can be identified as remnants of units  of
the plateau sequence.)

Hesperian System

The  Hesperian  System records extensive evidence  of  volcanism,
tectonism,  and  canyon and channel formation that is  relatively
unscathed by impact craters and other severe degradation.  Crater
size-frequency  distributions  for  pristine  Hesperian  surfaces
closely follow a -2 power law for craters larger than about 2  km
in  diameter  (Neukum  and  Wise,  1976).  This  relation  allows
comparison  of  crater  ages determined  for  different  diameter
ranges.  Densities  of either 2- or 5-km craters were  determined
for  most  Hesperian surfaces and are used to designate  relative
ages on the correlation chart,  although some surfaces have  been
degraded to the extent that densities of craters larger than 5 km
in  diameter  more  accurately represent the crater  age  of  the
surface.  Hesperian  materials  cover highland terrain  in  thick
expansive sheets and also as patches of intercrater plains;  they
partly  cover  the northern plains and floors of  chasmata.  They
form extensive volcanic flows in the Tharsis Montes, Alba Patera,
Syria  Planum,  and Tempe Terra regions.  Most  Amazonian  plains
materials  are  surrounded  by or contain  windows  of  Hesperian
materials.  We  estimate that more than half of the map area  was
resurfaced during the Hesperian Period.

Younger  fractured  material  (unit Hf) is exposed  in  Claritas,
Thaumasia,  and  Ulysses Fossae and in areas adjacent  to  Valles
Marineris,  Syria Planum,  and Uranius Patera.  Although the unit
intergrades  locally with the older fractured material (unit Nf),
it  is  cut by fewer faults of  less  complexity.  The  Hesperian
fractured  unit  may  have been partly resurfaced by  lava  flows
during later stages of less intense tectonic activity.

The  most  extensive unit in the  Hesperian  System,  the  ridged
plains  material (unit Hr),  is characterized by smooth  surfaces
with widely spaced, long, sinuous wrinkle ridges similar to those
of  the lunar maria.  The unit is interpreted to consist of  lava
flows (Scott and Carr, 1978; Greeley and Spudis, 1981). It is the
basal rock-stratigraphic unit of the Hesperian System on both the
Mariner 9 map of Mars (Scott and Carr, 1978) and the present map.
Ridged  plains  material  occurs  in some lowland  areas  and  is
widespread  throughout  the highlands,  particularly  from  Lunae
Planum to Solis Planum, where its western margin is buried by the
lower member (unit Hsl) of the Syria Planum Formation. It is also
exposed in scattered patches around the south end of the  Tharsis
swell,  forming  a broad arc that extends into Amazonis Planitia.
Northwest  of  Kasei Valles,  a major  disconformity  is  clearly
marked  by the overlap and truncation of faults and fractures  in
the  ridged  plains  unit  by  younger  flows  from  the  Tharsis
volcanoes. This boundary, more than 400 km long, is nearly linear
and may be fault controlled.

Most  of  the Tempe Terra Formation consists  of  smooth,  nearly
featureless material.  Low-relief, overlapping lobate scarps that
may be lava-flow edges are visible in all members.  The formation
occurs  within  the fractured uplands of Tempe  Terra,  including
Tempe and Mareotis Fossae. Its three members (units Htl, Htm, and
Htu)  probably  consist of lava flows erupted from  small  shield
volcanoes,   fissures,   and   circular  to   elongate   collapse
depressions that are common throughout the outcrop area. The lava
flows  blanket  and  embay cratered plains and  highly  fractured
terrain  of  Noachian age;  they are in turn embayed  by  younger
flows.

Within  the Valles Marineris and adjacent chasmata are mesas  and
rounded  hills  that  consist  of  alternating  light  and   dark
horizontal  beds  of the layered member (unit Hvl) of the  Valles
Marineris interior deposits. The tops of these eroded remnants in
places approach the level of the surrounding plateau,  suggesting
that the layered materials once filled the canyons. The origin of
the  unit is uncertain.  According to one  hypothesis  (McCauley,
1978),  the  layered materials accumulated as waterlaid sediments
in large lakes within the canyons;  their deposition was followed
by  episodes  of  catastrophic  draining  throughout  the  canyon
systems.  The lake sediments may consist of materials eroded from
canyon walls,  eolian material, and volcanic ash from the Tharsis
volcanoes.   Alternatively,  Peterson  (1981)  has  ascribed  the
occurrence of layered deposits in Hebes Chasma (just north of the
main  Marineris  system) to pyroclastic infilling of  the  chasma
from sources beneath its floor.

In  the western volcanic assemblage,  crater  densities  indicate
about  the same age for the lower members (units  Hal,  Ht1,  and
Ht2)  of  the Alba Patera and Tharsis Montes Formations  and  the
lower  and upper members (units Hsl and Hsu) of the Syria  Planum
Formation.  They  are  clearly identified as lava flows by  their
morphology,  as  are the upper members of these  formations.  The
lowermost  member of the Alba Patera Formation  nearly  encircles
the  volcano and extends as far as 1,500 km from its  center;  it
embays  the  older fractured material (unit Nf) around  the  east
edge  of  Acheron  Fossae.  Elsewhere the outer boundary  of  the
lowermost  member is not clearly defined except where the  member
is overlapped by the plains member (unit Aop) of the Olympus Mons
Formation (lat 23 N., long 123). Many of the lower, Hesperian-age
flows  of the Tharsis Montes Formation probably  originated  from
fissures  on  the lower slopes of Arsia Mons and from around  the
base  of Uranius Patera.  They occur as far as 2,000 km from  the
Tharsis  Montes and have more subdued relief and generally  lower
elevations than the younger members of the formation.  The  lower
member  of  the  Syria  Planum Formation  appears  to  have  been
extruded from fissures,  whereas the upper member probably issued
from fissures and also from partly buried calderas near the crest
of Syria Planum.

The older channel, flood-plain, and chaotic materials (units Hch,
Hchp,  and  Hcht) are widespread,  mostly northeast of the Valles
Marineris.   Martian  outflow  channels  are  commonly   sinuous,
braided,   many  kilometers  wide,   and  more  than  a  thousand
kilometers  long.   Most  originate  from  chasmata  and  chaotic
material (unit Hcht), although the Mangala Valles, in the western
part of the map area, appear to head at a fissure. Channel floors
are marked by striations,  terraces,  and teardrop-shaped bars or
islands (map symbol "b").  Outflow channels that debouch onto the
floor  of  Chryse  Planitia are mostly  obscured  by  flood-plain
material  and  younger lava flows of the lowermost  member  (unit
Aa1)  of  the Arcadia Formation.  Crescentic depressions that  in
size  and shape closely resemble meander patterns of  terrestrial
rivers  are recognized within the channel banks of  Ares  Vallis.
These  depressions in isolated exposures of flood-plain  material
occur in Acidalia Planitia as far north as lat 45 N.,  suggesting
that  flooding  from the outflow channels was  extensive  (Scott,
1982).   The  channel  and  adjacent  flood-plain  materials  are
overlapped  by members (units At4 and At5) of the Tharsis  Montes
Formation in the Kasei Valles, and by the floor member (unit Avf)
of  the Valles Marineris interior deposits in Echus Chasma.  Most
of  the  channels cut ridged plains material (unit Hr)  in  Lunae
Planum and Chryse Planitia and some cut the layered member  (unit
Hvl)  of  the  Valles Marineris interior deposits  within  Gangis
Chasma;  thus the channels are largely middle to upper  Hesperian
in  age.  Crater  densities  of scoured channel  and  flood-plain
materials  south  of Chryse Planitia generally confirm  this  age
designation (Carr and Clow,  1981);  an exception may be  channel
material in Mawrth Vallis (lat 22 N.,  long 12 to 20),  for which
crater counts (Masursky and others, 1980) suggest a late Noachian
to early Hesperian age.

The  formation of the channels has been attributed to erosion  by
running  water  by most authors,  for example,  Baker and  Milton
(1974) and the Mars Channel Working Group (1983); other suggested
processes include wind erosion (Cutts, 1973), lava erosion (Carr,
1974),  and  glaciation (Lucchitta and  others,  1981).  However,
sufficient water may not have been available to erode the  deeper
channels of Kasei,  Ares, and Shalbatana Valles (Sharp and Malin,
1975).   These   workers  suggested  that  the  deeper   channels
originated  by  tectonic subsidence and by sublimation of  ground
ice,  which  produced  incipient linear depressions  that  rivers
later eroded into channels.  Various processes have been proposed
to account for the release of water.  Masursky and others  (1977)
suggested  that  large  volumes of water may have come  from  the
melting of interstitial ice in the subsurface because of volcanic
heating.  Soderblom  and  Wenner (1978) proposed that  scarps  or
small  channels  retreating  headward  occasionally   intercepted
subsurface  fluid  reservoirs,  causing sudden increases in  flow
rates.  Carr (1979) postulated that artesian conditions developed
beneath large craters in the low region south of Chryse Planitia,
allowing the rapid release of huge volumes of water.

The Vastitas Borealis Formation contains the oldest rocks in  the
northern plains assemblage. Its four members are distinguished on
the  basis  of  secondary  morphologic  characteristics  and  are
degraded to varied degrees; craters smaller than 5 km in diameter
have  been  largely  obliterated.  Densities  of  larger  craters
indicate that the lowermost member is middle Hesperian in age and
that the other members are late Hesperian. The members intergrade
with and do not appear to overlap one another. Large areas of the
formation  have  many conical hills whose crests are darker  than
their  flanks;   some  hills  have  summit  craters  and  may  be
volcanoes.  Where  the hills are closely spaced or coalesce  into
clusters,  they are mapped as the knobby member (unit  Hvk).  The
ridged member (Hvr) is characterized by ridges that commonly form
whorled  patterns  resembling fingerprints.  The  grooved  member
(unit  Hvg)  consists of irregular troughs,  some of  which  form
polygons  as  much  as 20 km across.  Whether  these  ridges  and
grooves  are  primary  or  secondary features  is  not  resolved;
suggested origins include periglacial,  tectonic,  and compaction
processes (Carr and Schaber, 1977; Pechmann, 1980; McGill, 1985).
The mottled member (unit Hvm) forms an extensive belt that nearly
encircles  the northern part of the planet between about  lat  50
and  70 N.  (Tanaka and Scott,  unpub.  data,  1985).  Mariner  9
pictures  of this belt were degraded by atmospheric haze and high
sun  angles  that combined to blur the surface and  produce  high
albedo contrasts.  Viking images show that the contrasting  light
and dark patches are due chiefly to bright crater-ejecta blankets
superposed  on dark intercrater areas (Witbeck,  1984,  p.  296).
Stratigraphic relations between the Vastitas Borealis and Arcadia
Formations  are unclear except in Acidalia  Planitia,  where  the
lowermost  member (unit Aa1) of the Arcadia overlies the  grooved
and  mottled  members of the Vastitas Borealis.  Because  of  the
extensive  deformation  and surface degradation of  the  Vastitas
Borealis Formation,  its primary composition is uncertain, but it
may  include lava flows,  fluvial deposits,  and eolian  material
within a permafrost zone.

Amazonian System

The Amazonian System as originally defined (Scott and Carr, 1978)
was considered to include relatively young, featureless materials
covering  most of the lowlands in Amazonis Planitia.  On the low-
resolution Mariner 9 images,  these plains appear flat to  gently
rolling  and  have  few  topographic  irregularities.  They  were
interpreted  to  consist largely of lava flows covered by  varied
amounts  of  eolian  material;  on  the  basis  of  rather  broad
morphologic variations, their materials were classified as smooth
plains,  cratered plains,  and volcanic plains of Tharsis Montes.
On  the present map,  these materials and other young rock  units
have  been further subdivided and,  where  practicable,  assigned
formation names.

The  plains-forming  Arcadia  Formation  comprises  five  members
(units  Aa1  to  Aa5)  whose age  range  defines  and  spans  the
Amazonian Period (Scott and Carr,  1978).  All of the members are
exposed within Arcadia Planitia,  and are separated from the more
rugged,  plateau-forming  highlands to the south by the highland-
lowland  boundary scarp.  On Mariner 9 maps (Morris and  Dwornik,
1978;  Morris and Howard, 1981), the formation was interpreted to
consist of thick sequences of lava flows,  a conclusion supported
by  the  present  mapping.  The common boundaries  of  the  older
members  are  poorly defined and,  in  places,  they  are  mapped
arbitrarily  on  the  basis of variations in  crater  density  or
slight  differences  in texture and albedo of the bounded  units.
Landforms  that  are commonly visible on  high-resolution  images
appear  to  be  lobate  flow  fronts,   pressure  ridges,   small
volcanoes, and collapsed lava tubes or lava channels.

The Medusae Fossae Formation occurs mostly north of the highland-
lowland  boundary  scarp between the highlands south  of  Medusae
Fossae  and  the lowlands of Amazonis Planitia.  It  consists  of
three members (units Aml,  Amm,  and Amu) deposited in horizontal
sheets;  no bedding is visible on high-resolution Viking  images.
Total thickness of the members may exceed 3 km,  according to the
most recent topographic maps of the region (S.S.C. Wu and Raymond
Jordan,  U.S. Geological Survey, unpub. data, 1985). The surfaces
of  the members are relatively smooth and flat to gently rolling;
they  have been etched and serrated by wind,  particularly  along
their  edges  (Ward,  1979).  The  deposits are  less  hilly  and
cratered  than those of the highlands,  but have more relief  and
are lighter colored than lava flows of the Arcadia Formation. The
Medusae  Fossae Formation has been provisionally  interpreted  to
consist  of ash flows (Malin,  1979).  Its surfaces resemble both
welded  and  nonwelded  ash-flow tuffs in  the  Basin  and  Range
province  of the Western United States (Scott and Tanaka,  1982).
Elongate  depressions  in  the upper member  may  represent  some
partly buried sources of the ash.  The lower member contains dark
resistant material that may be lava flows.

The  western  volcanic assemblage consists  of  relatively  young
materials  erupted  from and around large volcanoes  and  fissure
vents.  The  Amazonian-age units of the assemblage consist of the
upper three members of the Tharsis Montes Formation,  all of  the
Olympus  Mons  Formation,  and the upper two members of the  Alba
Patera Formation.  Member 3 (unit AHt3) of the Tharsis Montes and
the  entire  Ceraunius Fossae Formation straddle  the  Amazonian-
Hesperian boundary. (The Syria Planum Formation, also included in
the  assemblage,  is  entirely  of Hesperian  age  and  has  been
described above.)

The  Tharsis  Montes  Formation and its  giant  source  volcanoes
(Arsia,  Pavonis,  and Ascraeus Montes) constitute the best known
and one of the largest sequences of lava flows on Mars. The flows
are thinly spread across the broad,  gently arched Tharsis swell,
indicating  that  they had low viscosities and  yield  strengths,
high  eruption  rates,  and  a basaltic  composition  (Moore  and
others,  1978).  Relative  ages  of the members  were  determined
mainly by stratigraphic relations and in places by differences in
degradation  of the flows.  Crater counts on the various  members
verify  age relations and help to establish correlations  between
flows in some widely separated areas. Crater counts also indicate
that  eruption  of  lava flows of the  Tharsis  Montes  Formation
appears  to  have  been nearly continuous.  The  older  Hesperian
members  retain some morphologic features characteristic  of  the
younger  members (units At4,  At5,  and At6),  including  tongue-
shaped  flows with lobate fronts.  These  observations,  together
with  low crater densities and the relative absence of faults and
fractures  transecting  the flows,  indicate that  the  style  of
Tharsis  volcanism remained unchanged as tectonism declined  late
in Martian history.

The  Olympus Mons Formation consists of six members.  The  oldest
four  (units  Aoa1  to Aoa4) are aureole  deposits  of  uncertain
composition and origin.  They are broad,  flat, sheetlike, lobate
deposits  whose  surfaces  are  grooved,   ridged,  and  faulted.
Formation  of the aureoles is considered by most workers to  have
been  caused  by either volcanic or gravity-assisted  mechanisms.
Proposed  volcanic  origins  include  lava  flows  (McCauley  and
others, 1972), moberg ridges (Hodges and Moore, 1979), and welded
and nonwelded ash-flow tuffs (Morris,  1982). Postulated gravity-
assisted   processes  include  low-angle  thrusting  of   layered
material  from beneath the Olympus Mons  shield  (Harris,  1977),
gravity  sliding of shield flanks (Lopes and others,  1980),  and
gravity  spreading of shield flanks (Francis and  Wadge,  1983)--
perhaps with the aid of ground ice (Tanaka,  1985).  The ages  of
the aureoles relative to those of most other units in the Tharsis
region   can  be  only  broadly  determined.   The  aureoles  are
overlapped  by  the  postscarp shield member (unit  Aos)  of  the
Olympus  Mons  Formation that was extruded from  the  summit  and
flanks of the volcano. This member and the aureoles are, in turn,
buried  in  places by flows of the plains member (unit Aop)  that
originated  from  fissures  east of  the  shield.  The  lowermost
aureole member underlies member 4 of the Tharsis Montes Formation
and  member  3 of the Arcadia Formation  but  overlaps  fractured
materials  of Hesperian and Noachian age.  A few older flows from
Olympus  Mons are sharply truncated at the scarp or are  degraded
and  exposed  in  windows above  the  scarp.  Where  these  older
surfaces  exhibit  faults and grabens,  they have been mapped  as
younger fractured material (unit Hf).  Although crater counts  of
the  aureole  deposits  have been attempted (Hiller  and  others,
1982), their accuracy is questionable because mass wasting of the
rough and apparently soft surfaces of the aureoles promotes rapid
deterioration  of  crater  forms.   The  density  of   positively
identifiable impact craters on the aureoles is lower than that of
younger  materials that embay the aureoles  (Morris,  1982).  The
position  of the aureole deposits in the stratigraphic column  is
thus provisional.

Lava   flows  of  the  Ceraunius  Fossae  Formation  (unit  AHcf)
originate  from complex fracture systems that  extend  northeast-
southwest  across  a broad saddle between Ascraeus Mons and  Alba
Patera. The flows bury most of the fault fissures from which they
were  extruded  and  partly cover the lower member  of  the  Alba
Patera Formation. They are overlapped by the plains member of the
Olympus  Mons  Formation and by younger members  of  the  Tharsis
Montes Formation.

The  Alba Patera Formation ranges in age from Hesperian to  lower
Amazonian and contains three members. The upper member (unit Aau)
covers a depressed circular area as much as 600 km in diameter at
the  center  of the volcanic deposits.  It partly  buries  nested
calderas  at  the  crest  of the volcano and  covers  radial  and
concentric  faults of Alba and Tantalus Fossae that transect  the
middle (Aam) and lower (Hal) members of the formation.

Younger  channel  material (unit Ach)  and  flood-plain  material
(unit  Achp) occur (1) in a broad outflow channel that  debouches
into the western part of Amazonis Planitia;  (2) as small channel
deposits  in Arcadia Planitia on the lowermost member (unit  Aa1)
of the Arcadia Formation;  (3) as minor flood-plain deposits that
were  transported  down  graben valleys along the north  edge  of
Tempe  Fossae;  and  (4) associated with debris flows  in  Candor
Chaos.

Surficial  materials of various origins occur locally  throughout
the  map  area.  Slide  material (unit As) is  common  in  Valles
Marineris (Lucchitta, 1978, 1979) and along the northwestern part
of  the  basal  scarp  of  Olympus  Mons.  Origin  of  the  huge,
spatulate, concentrically ribbed deposits on the northwest flanks
of  the  Tharsis volcanoes is uncertain.  The surfaces  of  these
deposits are hummocky:  they exhibit many small hills and  closed
depressions  whose size decreases away from the deposits' source.
All of these features are characteristic of large volcanic-debris
avalanches  noted  in many volcanic regions  on  Earth  (Siebert,
1984).  The  concentric ridges also resemble recessional moraines
in Iceland and could have been emplaced on former local ice  caps
(Lucchitta,  1981). Eolian deposits (unit Ae) tend to concentrate
in  low areas where they mantle underlying terrain;  yardangs and
dunes are visible in some high-resolution pictures. Large patches
of  the  unit cover parts of the aureoles  around  Olympus  Mons.
Other  eolian  materials appear as bright or dark streaks in  the
lee of obstacles such as craters and hills,  although some of the
dark streaks may be areas swept free of loose debris (Thomas  and
Veverka,  1979).  The  floor  member  (unit Avf)  of  the  Valles
Marineris interior deposits probably consists of a combination of
volcanic,  eolian,  fluvial,  and  landslide fill derived in part
from erosion of the layered member (unit Hvl).

Unassigned materials

Isolated  mountains,   hills,  and  domes  occur  throughout  the
highlands, but as a group they are not assigned a position in the
stratigraphic column.  They may have been formed relatively early
in  Martian  history,  as they are commonly  embayed  by  younger
materials,  but  their small size does not permit reliable crater
counts. Those having features interpreted to be volcanic, such as
summit  depressions and flow patterns on their flanks and  around
their  bases,  are shown in red and are designated by the  symbol
"v".  Others  may be remnants of the basement complex or  of  the
hilly unit of the plateau sequence; these are shown in dark brown
and are designated by the symbol "m".

Mantling deposits of volcanic,  eolian,  and alluvial origin form
the  smooth plains that cover the floors of many  craters.  These
deposits  are generally more common and thicker in older  craters
than  in younger ones.  Because their crater densities indicate a
wide age range, they are not shown on the correlation chart; they
may be mostly Amazonian in age.  The mantling deposits are  shown
in orange and designated by the symbol "s".

STRUCTURAL HISTORY

The   tectonic  history  of  the  western  equatorial  region  is
reconstructed  from transection relations among various  tectonic
features  and the stratigraphic  units.  Intermediate- and  late-
stage  tectonism  in  the  map  area  is  mostly  related  to  or
associated  with  the Tharsis swell.  Older  regional  structural
patterns  in  the  map  area  may be  related  to  early  Tharsis
tectonism,  but  this relation has yet to  be  established.  Such
structural   patterns  include:   (1)  fault  sets  in  Claritas,
Thaumasia,  Coracis,  Melas,  and  Nectaris Fossae  that  predate
radial Tharsis faulting; (2) a broad, Tharsis-centered arc formed
by old volcanoes in Terra Sirenum (Scott and Tanaka,  1981);  (3)
the semicircular Acheron Fossae arch containing concentric faults
and  volcanic  structures;  and  (4) possible  ancient  fractures
associated   with  initial  structural  development   of   Valles
Marineris (Masson, 1976).

Radar  profiles  (Downs  and  others,  1982)  indicate  that  the
northeast-trending,  elliptically  shaped Tharsis swell may be as
much as 7 km higher than the surrounding terrain. Theories of its
formation  are  speculative;  proposed models  include  isostatic
uplift followed by flexural loading (Phillips and  others,  1973;
Banerdt and others,  1982), thick accumulations of volcanic flows
extruded  through  a locally thin lithosphere (Solomon and  Head,
1982),   and  crustal  thickening  by  intrusion  (Willemann  and
Turcotte,  1982).  Our  mapping suggests that development of  the
Tharsis  swell involved a complex history of  episodic  tectonism
and  volcanism  on local and regional scales.  The  most  intense
deformation  around  Tharsis  occurred during  the  Noachian  and
Hesperian  Periods,  resulting in fault systems that trend mostly
northeast to north but also radial to a sequence of centers  that
shifted from Syria Planum to Pavonis Mons (Wise and others, 1979;
Plescia  and  Saunders,  1982).  These faults are visible in  the
highly  deformed  older fractured material and  basement  complex
(units Nf and Nb) of  Thaumasia,  Claritas,  Ceraunius,  Acheron,
Mareotis,  and  Tempe Fossae;  in the younger fractured  material
(unit  Hf);  and  in  the lava flows of the  Alba  Patera,  Syria
Planum,  and Tempe Terra Formations. The radial fractures deviate
around local circular structures that include Alba Patera,  Syria
Planum, and features on the Tempe Terra plateau.

As the intensity of Tharsis faulting diminished,  wrinkle  ridges
similar  in  morphology  to lunar mare ridges  formed  on  smooth
plains  surfaces of Hesperian age;  prominent occurrences include
Solis,   Felis,   Sacra,  and  Xanthe  Dorsa.  The  ridges  trend
concentrically to the Tharsis swell within a wide belt  extending
several thousand kilometers from its center.  Most ridges predate
the  formation  of  Kasei and Maja Valles  and  postdate  intense
faulting  in the Thaumasia and Tempe Fossae regions.  They may be
either   nearly   parallel  or  normal  to  faults   and   linear
depressions;  some grade into large,  linear edifices interpreted
to be volcanoes,  as at lat 46 S.,  long 172. Proposed origins of
ridges include:  eruption of volcanic material along structurally
controlled   linear   trends   (Greeley   and   others,    1977),
compressional folding associated with the Tharsis swell (Wise and
others,  1979;  Watters  and Maxwell,  1983),  and  compressional
folding  caused by contraction of the planet's surface  (Gifford,
1981, p. 322).

Most  development  of  the Valles  Marineris  canyon  system  and
associated  outflow channels and chaotic terrain occurred  during
the Hesperian Period. The canyons are incised in a thick stack of
plateau  sequence  rocks,  capped  around their western  part  by
ridged   plains   material  and  the  Syria   Planum   Formation.
Significant   canyon-scarp  retreat  appears  to  have   followed
emplacement  of these cap rocks,  but it is not known if  all  of
these  units  were  breached  by the  developing  canyon  system.
Possibly the ridged plains material of Lunae and Solis Plana  was
in part lava flows extruded from fault fissures that later formed
Valles  Marineris.  Compared with Noctis Labyrinthus,  the  large
canyons  of  Valles  Marineris  are  more  highly  developed  and
probably older,  possibly early Hesperian.  Proposed processes of
canyon  formation  include ice sublimation (McCauley and  others,
1972),  crustal rifting (Sharp,  1973; Frey, 1979), and extrusion
or relocation of huge volumes of underlying magma  (Sharp,  1973;
Schonfeld,  1979).  Eolian  and fluvial excavation of the central
Valles  Marineris  canyons was minor at most  (Sharp  and  Malin,
1975).  The  following  sequence of formation of the canyons  and
associated  channels  is  suggested by  the  additional  geologic
information  obtained  by our mapping.  (1)  Deep-seated  heating
resulted  in crustal expansion and rifting analogous to the  East
African  rift  system on Earth,  but the  thick  and  homogeneous
Martian  crust  was cut by rifts that are broader and  straighter
than  those on Earth (Frey,  1979).  Rift valleys attained  about
half their present size from adjustment to regional  uplift.  (2)
Possible  concurrent volcanism from rift faults formed the ridged
lavas in Lunae, Solis, and Sinai Plana. Withdrawal of lava caused
further  canyon subsidence.  (3) Layered deposits  were  emplaced
either  in lakes that filled the canyons (McCauley,  1978) or  as
ash-fall deposits (Peterson,  1981).  (4) Outflow channels formed
from  catastrophic  release  of  water  from  some  chasma  lakes
(McCauley,  1978) or aquifers (Soderblom and Wenner,  1978;  Carr
and Clow,  1981). (5) Canyons continued to expand by faulting and
landsliding.  Eolian  and fluvial erosion removed and transported
most layered deposits.

Infilling  of  the  northern  plains  apparently  commenced  with
emplacement  of the ridged plains material  (unit  Hr),  although
evidence  of possible earlier deposits may have been destroyed by
extensive  erosion  along the  highland-lowland  boundary.  Scarp
retreat along this boundary is evidenced by knobby remnants (unit
HNu) of the highland plateau sequence that occur several  hundred
kilometers  north  of  the boundary's  present  position.  Ridged
plains  material is overlain in Chryse Planitia by channel  (unit
Hch)  and  flood-plain  (unit Hchp) deposits  and  the  lowermost
member (unit Aa1) of the Arcadia Formation.  Several explanations
for  the  early  formation of the  northern  lowlands  have  been
postulated,  including  (1) breakup of the crust due either to  a
volume-expanding  phase change in the mantle (Mutch and Saunders,
1976,  p.  51-53) or to mantle convection (Wise and others, 1979)
and  (2) impact by an asteroid-size body (Wilhelms  and  Squyres,
1984).

Following  formation  of the northern plains,  Tharsis  tectonism
revived in Amazonian time,  but at much lower intensity,  causing
sporadic  faulting  in parts of the ridged  plains  material  and
plateau sequence rocks and in the Tharsis Montes, Medusae Fossae,
Alba Patera,  Arcadia,  Tempe Terra, and Olympus Mons Formations.
Late-stage  calderas and concentric grabens formed on many of the
Tharsis  volcanic structures.  Gravity studies suggest  that  the
Tharsis swell is at least partly compensated at present (Phillips
and others,  1973). Complete isostatic compensation of Tharsis at
great  depth has been simulated by crustal-thickness and  mantle-
density models (Sleep and Phillips, 1979).

Other structural features occur locally in the western equatorial
region.  Ancient  impact  basins  are mostly  buried  and  highly
degraded  but have exerted structural and topographic control  on
channels,  ridges,  linear  depressions,  and  perhaps  on  other
features (Schultz and others,  1982).  Relatively recent vertical
faulting  and expansion within the Valles Marineris canyon system
have offset canyon-wall and -floor materials (Blasius and others,
1977).  The  scarp (indicated by fault symbols) along  the  north
edge  of Argyre Planitia probably formed during or soon after the
impact that formed this basin.  Such scarps that encircle  impact
basins  are  common  on the Moon.  Origin of the basal  scarp  of
Olympus  Mons is controversial;  hypotheses  include  deep-seated
vertical  tectonism  (Mutch and others,  1976,  p.  189-190)  and
landslide scars (Lopes and others, 1980).

GEOLOGIC SUMMARY

1. Early Noachian--High meteorite flux and intense bombardment of
Martian surface;  rugged basement rocks,  mountains,  and  Argyre
Basin  formed;  northern  lowlands  developed  by  processes  yet
unknown.

2.   Middle  Noachian--Decreasing  impact  rate;  emplacement  of
volcanic and impact-breccia plateau material;  extensive faulting
formed Claritas, Coracis, Acheron, Melas, and Nectaris Fossae.

3.  Late Noachian--Widespread resurfacing and intercrater filling
partly subdued older surfaces; formation of large ridges in Terra
Sirenum and Noachis Terra; faulting radial to Syria Planum formed
Ceraunius,  Tempe, Mareotis, and Noctis Fossae; highland surfaces
channelled and etched.

4.  Early  Hesperian--Eruption  of lava flows (later  ridged)  in
intercrater and lowland plains and in Lunae Planum; long, tongue-
shaped  lava  flows  emplaced east of Alba Patera  and  at  Tempe
Terra;  faulting  and rifting dominantly radial to Syria  Planum-
Pavonis  Mons  formed  Valles Marineris  and  Ulysses,  Memnonia,
Sirenum,   Icaria,  Thaumasia,  and  Fortuna  Fossae;  widespread
degradation  and  burial of most older cratered terrain  by  lava
flows in lowland region.

5.  Late Hesperian--Extensive lava flows erupted from sources  at
Tharsis Montes,  Alba Patera,  Ceraunius Fossae, Tempe Terra, and
Syria  Planum;   deposition  of  layered  material  in  chasmata;
emplacement  of  lavas  or  sediments in  northern  Acidalia  and
Arcadia Planitiae;  faulting of Noctis Labyrinthus;  formation of
chaotic  terrain  and large outflow channels north  and  east  of
Valles Marineris,  extending into flood plains in Chryse Planitia
and in southern Acidalia Planitia.

6.  Early  Amazonian--Extrusion  of  younger lava  flows  on  and
surrounding the Tharsis Montes and Alba Patera and in lowlands of
Amazonis,  Arcadia,  and Acidalia Planitiae;  formation of oldest
Olympus Mons aureoles.

7.  Middle  Amazonian-- Flows  emplaced  around  Tharsis  Montes;
eruption  of lavas continued in Amazonis Planitia;  deposition of
soft-appearing  material in the Memnonia Sulci and Gordii  Dorsum
areas.

8.  Late  Amazonian--Ribbed  debris aprons  formed  on  northwest
flanks  of  the Tharsis Montes and Olympus Mons,  accompanied  by
minor volcanism;  channel deposits in western Amazonis  Planitia;
deposition of plains material in Arcadia Planitia;  deposition of
soft  material  continued  in Medusae Fossae and  Amazonis  Sulci
areas.

REFERENCES CITED

Baker,  V.R.,  and Milton,  D.J.,  1974,  Erosion by catastrophic
floods on Mars and Earth: Icarus, v. 23, no. 1, p. 27-41.

Banerdt,  W.B.,  Phillips, R.J., Sleep, N.H., and Saunders, R.S.,
1982, Thick shell tectonics on one-plate planets: Applications to
Mars:  Journal of Geophysical Research,  v. 87, no. B12, p. 9723-
9733.

Blasius,  K.R.,  Cutts,  J.A., Guest, J.E., and Masursky, Harold,
1977,  Geology of the Valles Marineris: First analysis of imaging
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DESCRIPTION OF MAP UNITS

Although the origin and composition of many units are obscure  or
controversial,  their  interpretations  are  based  on  objective
descriptions  of  morphologic characteristics visible  on  Viking
photomosaics and images.

LOWLAND TERRAIN MATERIALS

Consist  of all plains-forming units between the highland-lowland
boundary  scarp  and  the north edge of  the  map,  exclusive  of
materials  of  the  western volcanic assemblage  on  the  Tharsis
swell.

Northern plains assemblage

Materials  deposited  in widespread sheets  on  northern  plains.
Within  each  formation,  members  mapped at places on  basis  of
crater  density;   these  contacts  are  approximately   located.
Assemblage  postdates  highland-lowland  boundary  scarp  (Scott,
1979).

ARCADIA  FORMATION--Forms low-lying plains in Arcadia,  Amazonis,
and Acidalia Planitiae. Embays highland margins and partly buries
outflow  channels of Kasei,  Shalbatana,  Simud,  Tiu,  and  Ares
Valles. Members distinguished on basis of morphology, albedo, and
crater  density;   common  boundaries  of  older  members  mapped
arbitrarily at places.  Flows with lobate margins and small hills
with  summit  craters visible  in  many  places.  High-resolution
pictures  show  that  sources of some flows  are  small  cratered
cones. Interpretation: Mostly lava flows and small volcanoes

Aa5  Member  5--Relatively  small  areal  extent.   Dark,  fresh-
appearing flows; few superposed impact craters. Type area: lat 47
N., long 30

Aa4  Member  4--In  Arcadia Planitia underlies member 5  and  has
similar  appearance;  one other occurrence in channel  system  of
Chryse Planitia. Type area: lat 45 N., long 175

Aa3  Member 3--Forms smooth plains west of Olympus Mons aureoles;
embays  both the aureoles and fractured terra of Acheron  Fossae.
Flow fronts visible in places. Type area: lat 15 N., long 155

Aa2 Member 2--Underlies members 3,  4, and 5 in Arcadia Planitia.
Includes  many  small  (<10-km-diameter)  structures   resembling
volcanoes  and cinder cones.  Curved concentric ridges visible on
surfaces of flows. Type area: lat 45 N., long 155

Aa1 Member 1--Widespread in Chryse and Amazonis Planitiae.  Mare-
type (wrinkle) ridges common. Type area: lat 30 N., long 40

MEDUSAE FOSSAE FORMATION--Consists of extensive,  relatively flat
sheets,  generally  smooth  to  grooved  and  gently  undulating;
deposits appear to vary from soft to indurated;  albedo moderate.
Occurs near equator in western part of map area.  Total thickness
may exceed 3 km

Amu  Upper member--Discontinuous but widespread  deposits  extend
from  south of Olympus Mons westward across Amazonis Sulci to map
boundary.   Smooth,  flat  to  rolling,  light-colored  surfaces;
sculptured  into  ridges  and grooves in places  (as  in  Medusae
Fossae); broadly curved margins, locally serrated. Type area: lat
0 N.,  long 160.  Interpretation:  Nonwelded ash-flow or ash-fall
tuff  or  thick  accumulation  of  eolian  debris;  wind  eroded,
particularly along margins

Amm  Middle member--Similar to upper member but in places (as  at
Memnonia Sulci) surface appears rougher,  more deeply eroded; cut
by  scarps  of  Gordii  Dorsum and transected  in  type  area  by
intersecting  joint  sets.  Type  area:  lat  10  N.,  long  160.
Interpretation:  Welded and nonwelded pyroclastic rocks or layers
of relatively soft to indurated eolian deposits

Aml  Lower  member--Two small occurrences in  western  map  area.
Surfaces smooth to rough and highly eroded,  darker than those of
other members.  Type area:  lat 0 N.,  long 174.  Interpretation:
Lava flows interbedded with pyroclastic rocks or eolian deposits

VASTITAS BOREALIS FORMATION--Subpolar plains deposits of northern
lowlands.  Its  four members distinguished on basis of morphology
or albedo contrast; placement of contacts locally arbitrary

Hvm  Mottled  member--Major  occurrence  north  of  map  boundary
(Tanaka and Scott,  unpub.  data,  1985); extends as far south as
topographic  reentrants  between Acidalia  and  Chryse  Planitiae
where  appears  windswept.  Crater-ejecta  blankets  have  higher
albedo  than adjacent terrain;  lobate flow fronts visible;  some
small   hills  present.   Type  area:   lat  55  N.,   long   40.
Interpretation:  Possibly  consists  of lava flows  erupted  from
fissures and small volcanoes or of alluvial and eolian  deposits.
Mottled  appearance due to contrast between generally low  albedo
of plains and brightness of small hills and impact-crater aprons

Hvg   Grooved  member--Similar  to  mottled  member  in  Acidalia
Planitia  but  marked by curvilinear and  polygonal  patterns  of
grooves and troughs; closed polygons as wide as 20 km. Type area:
lat  45 N.,  long 15.  Interpretation:  Material same as  mottled
member;  patterns  may  be  due to compaction or to  tectonic  or
ground-ice phenomena

Hvr  Ridged  member--Three small occurrences  of  mottled  plains
characterized by concentric pattern of low, narrow ridges about 1
to 2 km wide.  Type area:  lat 38 N., long 33; other two outcrops
near  lat  54 N.,  long 176.  Interpretation:  Material  same  as
mottled  member;  origin  of  ridges  unknown  but  they  may  be
periglacial structures or channel-meander features accentuated by
differential erosion

Hvk  Knobby  member--Similar in appearance to mottled member  but
generally has higher albedo and abundant  small,  dark,  knoblike
hills,  some with summit craters.  Type area:  lat 55 N., long 5.
Interpretation:  Material  same as mottled member;  hills may  be
small volcanoes or remnants of highland terrain or crater rims

Channel-system materials

Deposited  in outflow channels and on flood plains;  exhibit both
depositional and erosional features.  Interpretation: Channel and
flood-plain   materials  of  alluvial   origin;   some   surfaces
sculptured by flood waters. Chaotic material formed by disruption
of terrain by ground-water release.

Ach/Achp Younger channel and flood-plain materials--Along western
margin of map area,  form plain as wide as 200 km marked by dark,
sinuous,  intertwining channels with bars and islands; fill small
channels  in Arcadia Planitia,  along north edge of Tempe Fossae,
and in Ophir and Candor Chasmata. Crater counts and superposition
relations indicate Amazonian age. Type areas: lat 15 N., long 177
(unit Ach) and lat 22 N., long 171 (unit Achp)

Hch/Hchp/Hcht Older channel, flood-plain, and chaotic materials--
Mainly  between  Valles Marineris and Chryse  Planitia,  also  in
other   highland  locations.   Channel  deposits   longitudinally
striated;  teardrop-shaped channel bars large and well developed.
Flood-plain  material occurs adjacent to channels and in  lowland
plains  below channel mouths;  smooth  and  featureless.  Chaotic
material occurs at source areas and along margins of channels and
within some chasmata and craters;  generally a mosaic of highland
blocks in depressions. Type areas: lat 25 N., long 60 (unit Hch);
lat 27 N., long 53 (unit Hchp); lat 5 S., long 27 (unit Hcht)

HIGHLAND TERRAIN MATERIALS

Rock and rock-tectonic units of moderate to high relief; dominate
southern   and  near-equatorial  parts  of  map  area.   Volcanic
mountains and associated lava flows of Tharsis  region,  although
not   typical   of  highland  terrain,   are  included  in   this
physiographic  classification  because  they  are  superposed  on
highland   terrain  or  form  high  plains  and  locally   rugged
topographic features.

Western volcanic assemblage

Volcanoes  and lava flows in Tharsis region of Mars (Schaber  and
others, 1978; Scott and others, 1981).

THARSIS  MONTES  FORMATION--Includes large volcanic  shields  and
associated lava flows of Arsia Mons,  Pavonis Mons,  and Ascraeus
Mons;  lava  flows similar in morphology to  terrestrial  basalts
(Schaber and others, 1978)

At6 Member 6--Fresh-appearing lava flows form smooth,  fan-shaped
arrays on flanks of Arsia,  Pavonis,  and Ascraeus Montes;  flows
probably  originate from fissures along major structural  trends.
Flanks  of  these  volcanoes exhibit  grabens,  some  concentric.
Member also includes most recent fill within central calderas  of
Tharsis Montes. Type area: lat 5 S., long 117

At5   Member  5--Widespread  around  Tharsis  Montes   volcanoes.
Overlies  parts  of channel and flood-plain deposits (units  Hch,
Hchp)  of Kasei Valles;  contact with upper member (unit Hsu)  of
Syria Planum Formation northwest of Echus Chasma poorly resolved.
At places forms elongate,  light-colored flow lobes with abundant
dark wind streaks.  Cut by few faults. Type area: lat 20 S., long
120

At4 Member 4--Exposed mostly northeast and southwest of member 5.
Consists  of  overlapping  light flows  with  dark  wind  streaks
similar  to  those  of member 5;  flows elongate on  steep  upper
slopes,  broad  on gentler lower slopes.  High-resolution  images
show  pressure ridges concentric with lobate flow  fronts;  minor
faulting. Type area: lat 15 S., long 135

AHt3 Member 3--Makes up central shields of  Arsia,  Pavonis,  and
Ascraeus  Montes  and embays highland terrain west of Arsia  Mons
and along northwest side of Claritas Fossae, where light and dark
flows  common.  Fewer lobate fronts,  pressure ridges,  and  dark
streaks but more faults than in members 4 and 5.  Type area:  lat
27 S., long 127

Ht2  Member  2--Occurs  in southern  and  northeastern  parts  of
Tharsis region; embays highland terrain of Tempe Fossae. Composed
of relatively smooth flows having broad frontal lobes;  fractures
and faults common in places. Type area: lat 33 S., long 135

Ht1  Member  1--Scattered  outcrops in southern  Tharsis  region.
Generally  forms rough,  hummocky  surface;  mare-type  (wrinkle)
ridges in places; faults and fractures common locally. Type area:
lat 30 S., long 120

OLYMPUS  MONS FORMATION--Includes young lava flows extruded  from
fissures  in plains east of Olympus Mons,  young shield lavas  of
the  volcano,  and  aureole  deposits  surrounding  the  volcano.
Interpretation  of  aureoles:  Formed  by  gravity  spreading  of
materials forming a larger,  earlier Olympus Mons; alternatively,
could  be ash or lava flows (see text) Aop Plains  member--Embays
basal scarp of Olympus Mons and overlaps shield member.  Consists
of  many  overlapping  smooth lava flows ranging  in  shape  from
narrow  tongues to broad lobes;  flows appear to be extruded from
faults  and  fissures below scarp on southeast  side  of  Olympus
Mons. Type area: lat 20 N., long 125

Aos  Shield  member--Lava flows form  complex,  finely  textured,
interfingering tongues and lobes. Channels and levees extend down
flanks of Olympus Mons and across prominent basal scarp on north,
east,  and south sides;  collapse pits common.  Type area: lat 15
N., long 135

Aoa4  Aureole  member 4--Uppermost of a series of  aureole  units
around  Olympus  Mons that formed prior to  or  contemporaneously
with the volcano's basal scarp.  Forms broad,  semicircular, flat
lobes;  corrugated, cut by numerous faults that formed scarps and
deep troughs and grabens. Type area: lat 25 N., long 145

Aoa3 Aureole member 3--Forms two lobes; similar to but underlying
member 4. Type area: lat 28 N., long 134

Aoa2  Aureole member 2--Forms three lobes;  similar to members  3
and  4;  underlies member 3 in relatively small area on southwest
side of Olympus Mons;  on east side occurs as islands  surrounded
by plains member. Type area: lat 14 N., long 143

Aoa1  Aureole member 1--Forms widespread basal aureole;  overlaps
younger  and  older  fractured  materials  (units  Hf  and   Nf).
Resembles  younger aureole members but smoother and more degraded
by wind. Type area: lat 15 N., long 147

AHcf  CERAUNIUS FOSSAE FORMATION--A series of  overlapping  flows
whose  surfaces  are relatively smooth and even toned to  mottled
and  streaked;  trends northeast across older fractured  material
(unit  Nf)  in Ceraunius Fossae;  channels with levees  occur  in
places.  Type area:  lat 23 N.,  long 115.  Interpretation:  Lava
flows, most of which originated from fissures

SYRIA  PLANUM FORMATION--Volcanic flows of intermediate age  that
originated  from crestal area and flanks of topographic  high  in
northern Syria Planum and Noctis Labyrinthus

Hsu  Upper  member--Partly encircles crest of  topographic  high.
Consists of both long narrow lava flows and sheet flows that have
prominent  lobes  mottled  light and dark;  pit  craters  common.
Relatively  smooth  around Valles Marineris.  Covers  some  fault
systems  of  Claritas  Fossae but cut by  few  others  in  Noctis
Labyrinthus. Flows probably extruded from calderas near summit of
Syria  Planum and from fissures;  gradational with lower  member.
Boundary  with  ridged  plains  material (unit  Hr)  not  clearly
defined, placed where member apears to overlap ridges. Type area:
lat 15 S., long 100

Hsl  Lower  member--Similar  to  upper  member  but  more  highly
cratered and faulted. Type area: lat 25 S., long 90

ALBA PATERA FORMATION--Forms low shield and extensive lava  flows
of Alba Patera volcano

Aau  Upper  member--Covers large area within and  around  central
calderas;  lava  flows  completely or partly bury most  ring  and
radial  structures;  channels with levees common along crests  of
flows. Type area: lat 40 N., long 110

Aam  Middle  member--Encircles crestal region and  upper  member;
occurs  as  low  radial ridges with summit channels and  as  long
narrow  tongues with leveed channels.  Highly faulted but  buries
many  grabens in older fractured material (unit Nf).  Type  area:
lat 40 N., long 118

Hal Lower member--Covers broad area around north and west  flanks
of  Alba Patera.  Flow fronts less distinct than those of younger
members.  Extends into plains region; appears to be overlapped in
places  by lowermost member (unit Aa1) of Arcadia  Formation  but
most of boundary indistinct. Type area: lat 35 N., long 125

Plateau and high plains assemblage

Forms ancient highland terrain and local tracts of younger deposits.

SURFICIAL DEPOSITS

As Slide material of three types--

(1)  In  Valles Marineris and other chasmata as rotational  slide
deposits from canyon walls;  surfaces hummocky near crown  scarp;
distal parts commonly lobate,  longitudinally striated, with long
runout.  Type area:  lat 9 S.,  long 72.  Interpretation: Gas- or
water-lubricated slides and debris flows

(2)  Mostly  on  northwest  flanks of Olympus  Mons  and  largest
volcanoes of Tharsis Montes as fanlike corrugated sheets as  wide
as  600 km that appear to override topographic obstacles  without
deflection of internal structure;  source areas hummocky, contain
small hills and circular depressions.  Type area:  lat 5 S., long
125.  Interpretation:  Volcanic-debris  avalanches resulting from
either slope failure or explosive volcanism;  alternatively,  may
be recessional moraines of former local ice caps

(3)  Along  scarps  of Charitum Montes  and  Mareotis  Fossae  as
smooth,  sloping aprons with steep,  high edges. No specific type
area.  Interpretation: Debris aprons produced by mass wasting and
slow, glacierlike flow of ice-rich plateau material

Ae  Eolian  deposits--Form broad level plains chiefly on  aureole
deposits of Olympus Mons.  Surfaces locally appear rough,  deeply
etched,  and  striated in directions of  prevailing  winds.  Type
area: lat 15 N., long 145

VALLES   MARINERIS  INTERIOR  DEPOSITS--Materials  within  Valles
Marineris  and  associated canyons,  excluding walls  and  mapped
landslides and channel materials

Avf  Floor  member--Occurs  along  canyon  floors  as  smooth  to
hummocky  and  rugged material of low relief.  No  specific  type
area. Interpretation: Mixture of landslides and debris flows from
canyon walls, eolian material, volcanic deposits, and channel and
possibly lacustrine deposits

Hvl  Layered  member--Thinly  bedded  dark  and  light  materials
visible in high-resolution pictures on floors of several canyons.
Forms platforms and hills of moderate to high relief.  Type area:
lat  7  S.,  long  48.  Interpretation:  Volcanic  or  lacustrine
material or both, deposited during intermediate to late stages of
canyon development

PLATEAU  SEQUENCE--Forms  rough,   hilly,   heavily  cratered  to
relatively  flat and smooth terrain covering most  of  highlands,
which   are  dominant  in  southern  hemisphere.   Several  units
represent   transitional   stages  modified   by   erosional   or
depositional processes

Hpl3   Smooth  unit--Forms  large  areas  of   flat,   relatively
featureless  plains in southern highlands;  locally embays  other
units  of  plateau sequence.  Faults and flow fronts  rare.  Type
area: lat 43 S., long 105. Interpretation: Thick interbedded lava
flows and eolian deposits that bury most of underlying rocks

Npl2  Subdued cratered unit--Forms plains (mostly  in  highlands)
marked by subdued and partly buried old crater rims.  Flow fronts
rare.  Type  area:  lat 28 S.,  long  162.  Interpretation:  Thin
interbedded  lava  flows  and eolian deposits  that  partly  bury
underlying rocks

Npl1  Cratered unit--Most widespread unit in southern  highlands;
locally  extensive in northern plains.  Highly  cratered,  uneven
surface of moderate relief; fractures, faults, and small channels
common. Type area: lat 45 S., long 148. Interpretation: Materials
formed  during period of high impact flux;  probably a mixture of
lava flows, pyroclastic material, and impact breccia

Npld  Dissected  unit--Similar in occurrence  and  appearance  to
cratered  unit  but more highly dissected by small  channels  and
troughs.  Type area:  lat 45 S.,  long 70. Interpretation: Origin
same  as  that  of  cratered  unit but  more  eroded  by  fluvial
processes

Nple Etched unit--Similar to cratered unit but deeply furrowed by
sinuous,  intersecting,  curved  to  flat-bottomed  grooves  that
produce an etched or sculptured surface.  Type area:  lat 45  N.,
long 55.  Interpretation: Cratered unit that has been degraded by
wind erosion, decay and collapse of ground ice, and minor fluvial
processes

Nplr  Ridged  unit--Resembles  ridged plains material  (unit  Hr)
where  units  adjoin,  but ridges generally  larger  and  farther
apart,  intervening areas rougher and more densely cratered. Type
area:  lat 15 S.,  long 163.  Interpretation:  Most ridges due to
normal   faulting  but  others  may  be  volcanic  constructs  or
compressional features

Nplh  Hilly unit--Rough,  hilly material that resembles  in  part
basement  complex  (unit Nb) and older fractured  material  (unit
Nf), but relief is gentler and faulting less intense. Type areas:
lat 12 S.,  long 174 and Nereidum and Charitum Montes surrounding
Argyre Planitia.  Interpretation: Ancient highland volcanic rocks
and   impact  breccia  uplifted  by  tectonism  and  impact-basin
formation during period of heavy bombardment

Hr  RIDGED  PLAINS MATERIAL--Major occurrences cover an  area  of
about  4,000,000 square km extending from Solis Planum  to  Tempe
Fossae.  Characterized  by broad planar surfaces with flow  lobes
visible in places and long, parallel, linear to sinuous mare-type
(wrinkle)  ridges;  ridges  about 30 to 70 km apart.  Type  area:
Lunae Planum, lat 10 N., long 65. Interpretation: Extensive flows
of  low-viscosity lava erupted from many sources at  high  rates;
ridges  either volcanic constructs or compressional features (see
text)

TEMPE TERRA FORMATION--Interpreted to consist of intermediate-age
lava flows extruded from small shield  volcanoes,  fissures,  and
depressions  on Tempe Terra plateau.  All members exhibit  lobate
scarps that may be edges of flows

Htu Upper member--Smooth,  light-colored, partly mottled material
that  embays  hilly,   mountainous,   and  fractured  terrain  of
highlands;  small  (<10-km-diameter) shield volcanoes visible  in
high-resolution pictures;  few faults and fractures; embayed by a
lower  member (unit Ht2) of Tharsis Montes Formation and by lower
member (unit Hal) of Alba Patera Formation. Type area: lat 36 N.,
long 86

Htm Middle member--Similar to upper member but faults, fractures,
and collapse depressions common. Type area: lat 42 N., long 80

Htl Lower member--Smooth to rough,  uneven surfaces; small faults
and  collapse  depressions common.  Overlaps hilly  and  cratered
units  (units Nplh,  Npl1) of plateau sequence but is embayed  by
upper and middle members of Tempe Terra Formation. Type area: lat
39 N., long 84

HIGHLY DEFORMED TERRAIN MATERIALS--The origin and composition  of
these  rock  units  are only surmised because  multiple  sets  of
fractures and grabens have obscured original characteristics. The
units  are interpreted to consist of impact breccia  interlayered
with volcanic flows and to intergrade locally

Hf   Younger  fractured  material--Occurs  mostly  around  Valles
Marineris  and  Syria and Sinai Plana and  northwest  of  Pavonis
Mons. Forms relatively smooth, raised surfaces of moderate relief
with fractures, grabens, and collapse depressions. Overlies other
highly  deformed  terrain  materials but is  embayed  and  partly
covered  by Syria Planum Formation and other younger rock  units.
Type area: lat 5 S., long 103

Nf   Older  fractured  material--Similar  to  younger   fractured
material  but  widespread,  has greater relief,  is  more  highly
deformed,  and faults are more complexly oriented;  impact-crater
outlines largely destroyed. Type area: lat 20 S., long 109

Nb  Basement complex--Undifferentiated material characterized  by
highly complicated structure and prominent relief. Most common in
Claritas and Mareotis Fossae areas.  Type area:  lat 28 S.,  long
100

HNu  UNDIVIDED MATERIAL--Forms hills and small knobs adjacent  to
highland-lowland  boundary scarp that extend almost to north  map
border.  Also  forms  walls  and  interior  mountains  in  Valles
Marineris  and  associated  canyons  and  channels,  as  well  as
hummocky  terrain  and scarps along edges of Chryse and  Acidalia
Planitiae.  No  specific  type  area.  Interpretation:  Erosional
remnants  and  exposures of  plateau  sequence,  highly  deformed
terrain materials,  ancient crater rims, and some other materials
that are older than surrounding rock units

EXPLANATION OF MAP SYMBOLS

b   TEARDROP-SHAPED  BAR  OR  ISLAND  ON  CHANNEL  FLOOR--May  be
erosional or depositional feature

c IMPACT CRATER MATERIALS--Yellow if superposed,  brown if partly
buried.  May  include rim crest (hachured),  central ring  (inner
circular  feature,  also hachured),  and central peak.  May  also
include secondary craters outside crater aprons.  Symbol "s"  and
orange  color  denote  smooth floor;  floor material  of  diverse
origins,  may be eolian or volcanic fill. Not mapped are material
of  superposed  craters less than about 100 km  across,  rims  of
partly buried craters less than about 150 km across,  and  smooth
crater floors less than about 30 km across

m MOUNTAIN--Relative age and origin unknown

v  VOLCANO--Relative  age unknown.  May have central caldera  and
radial channels