**GEOLOGIC MAP**

**OF THE**

**WESTERN EQUATORIAL REGION**

**OF MARS**

By

David H. Scott and Kenneth L. Tanaka

1986

Prepared for the National Aeronautics and Space Administration

by

U.S. Department of the Interior, U.S. Geological Survey

> (Published in hardcopy as USGS Miscellaneous Investigations Series Map
> I--1802--A, as part of the Atlas of Mars, 1:15,000,000 Geologic
> Series. Hardcopy is available for sale from U.S. Geological Survey,
> Information Services, Box 25286, Federal Center, Denver, CO 80225)

Please direct questions or comments about the digital version to:

> Richard Kozak
>
> U.S. Geological Survey
>
> 2255 N. Gemini Drive
>
> Flagstaff, AZ 86001
>
> e-mail: rkozak@flagmail.wr.usgs.gov
>
> **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 area, 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 l---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., Iong 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 appears 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

Slide material of three types---

As (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 material---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 material---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 southem 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 km2 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 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

**CONTACT**---Dashed where approximately located or gradational, or
where arbitrarily located based on crater densities of bounded map units

**FAULT OR GRABEN**---Bar and ball on downthrown side of fault

**MARE-TYPE (WRINKLE) RIDGE**---Symbol on ridge crest

**NARROW SINUOUS CHANNEL**---Wide age range

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

**DEPRESSION**

**CALDERA**

**JOINT OR FRACTURE PATTERN**

**HIGHLAND-LOWLAND BOUNDARY SCARP**---Diffuse zone of transition between
highland and lowland physiographic provinces; not shown where buried

**CREST OF BASAL SCARP OF OLYMPUS MONS**---Dashed where buried

**IMPACT CRATER MATERIALS**---Symbol \"**c**\" denotes crater-rim and
ejecta material; 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

**MOUNTAIN**---Relative age and origin unknown

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

**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
immeasurably 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,
1976). 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. The
youngest and best preserved impact basin on Mars, lying in the western
hemisphere, is more than 1,000 km across. It 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 toward 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 round 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 material* (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). Elongated
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 northeastsouthwest 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 early
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
material* (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 material
(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 gray
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, 1977).

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
mantel-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 impact
basin Planitia 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 from the
Viking 1 Orbiter primary mission: Journal of Geophysical Research, v.
82, no. 28, p. 4067--4091.

Carr, M.H., 1974, The role of lava erosion in the formation of lunar
rilles and Martian channels: Icarus, v. 22, no. 1, p. 1--23.

\_\_\_\_\_\_\_\_1979, Formation of Martian flood features by release of
water from confined aquifers: Journal of Geophysical Research, v. 84,
no. B6, p. 2995--3007.

Carr, M.H., and Clow, G.D.,1981, Martian channels and valleys: Their
characteristics, distribution, and age: Icarus, v.48, no.1, p. 91--117.

Carr, M.H., and Schaber, G.G., 1977, Martian permafrost features:
Journal of Geophysical Research, v. 82, no. 28, p. 4039--4054.

Chapman, C.R., and Jones, K.L., 1977, Cratering and obliteration history
of Mars: Annual Reviews of Earth and Planetary Science, v. 5, p.
515--540.

Cutts, J.A., 1973, Wind erosion in the Martian polar regions: Journal of
Geophysical Research, v. 78, no. 20, p. 4211--4221.

Downs, G.S., Mouginis-Mark, P.J., Zisk, S.H., and Thompson, T.W., 1982,
New radar-derived topography for the northern hemisphere of Mars:
Journal of Geophysical Research, v. 87, no. B12, p. 9747--9754.

Francis, P.W., and Wadge, Gerald, 1983, The Olympus Mons aureole:
Formation by gravitational spreading: Journal of Geophysical Research,
v. 88, no. B10, p. 8333--8344.

Frey, Herbert, 1979, Martian canyons and African rifts: Structural
comparisons and implications: Icarus, v. 37, no. 1, p. 142--155.

Gifford, A.W.,1981, Ridge systems on Mars, *in* Advances in Planetary
Geology: National Aeronautics and Space Administration Technical
Memorandum 84412, p. 219--363.

Greeley, Ronald, and Spudis, P.D., 1981, Volcanism on Mars: Reviews of
Geophysics and Space Physics v.19, no.1, p.13--41.

Greeley, Ronald, Theilig, Eilene, Guest, J.E., Carr, M.H., Masursky,
Harold, and Cutts, J.A., 1977, Geology of Chryse Planitia: Journal of
Geophysical Research, v. 82 no. 28, p. 4093--4109.

Gurnis, Michael, 1981, Martian cratering revisited: Implications for
early geologic evolution: Icarus, v. 48, no. 1, p. 62--75.

Harris, S.A., 1977, The aureole of Olympus Mons, Mars: Journal of
Geophysical Research, v. 82, no. 20, p. 3099--3107.

Hartmann, W.K., 1978, Martian cratering V: Toward an empirical Martian
chronology, and its implications: Geophysical Research Letters, v. 5,
no. 6, p. 450--452.

Hiller, K.H., Janle, Peter, Neukum, G.P.O., Guest, J.E., and Lopes,
R.M.C., 1982, Mars: Stratigraphy and gravimetry of Olympus Mons and its
aureoles: Journal of Geophysical Research, v. 87, no. B12, p.
9905--9915.

Hodges, C.A., 1980, Geologic map of the Argyre quadrangle of Mars: U.S.
Geological Survey Miscellaneous Investigations Series Map I-1181, scale
1:5,000,000.

Hodges, C.A., and Moore, H.J., 1979, The subglacial birth of Olympus
Mons and its aureoles: Journal of Geophysical Research, v. 84, no. B14,
p. 8061--8074.

Lopes, R.M.C., Guest, J.E., and Wilson, C.J., 1980, Origin of the
Olympus Mons aureole and perimeter scarp: The Moon and the Planets, v.
22, no. 2, p. 221--234.

Lucchitta, B.K., 1978, A large landslide on Mars: Geological Society of
America Bulletin, v. 89, no. 11, p. 1601--1609.

\_\_\_\_\_\_\_\_1979, Landslides in Valles Marineris, Mars: Journal of
Geophysical Research, v. 84, no. B14, p. 8097--8113.

\_\_\_\_\_\_\_\_1981, Mars and Earth: Comparison of cold-climate
features: Icarus, v. 45, no. 2, p. 264--303.

Lucchitta, B.K., Anderson, D.M., and Shoji, Hitoshi, 1981, Did ice
streams carve martian outflow channels?: Nature, v. 290, no. 5809, p.
759--763.

Malin, M.C., 1979, Mars: Evidence of indurated deposits of fine
materials \[abs.\]: National Aeronautics and Space Administration
Conference Publication 2072, p. 54.

Mars Channel Working Group, 1983, Channels and valleys on Mars:
Geological Society of America Bulletin, v. 94, no. 9, p. 1035--1054.

Masson, Philippe,1977, Structural pattern analysis of the Noctis
Labyrinthus-Valles Marineris regions of Mars: Icarus, v.30, no. 1, p.
49--62.

Masursky, Harold, Boyce, J.M., Dial, A.L., Jr., Schaber, G.G., and
Strobell, M.E., 1977, Classification and time of formation of Martian
channels based on Viking data: Journal of Geophysical Research, v. 82,
no. 28, p. 4016--4037.

Masursky, Harold, Dial, A.L., Jr., and Strobell, M.E., 1980, Martian
channels---A late Viking review \[abs.\], *in* Reports of the Planetary
Geology Program-1980, National Aeronautics and Space Administration
Technical Memorandum 82385, p. 184--187.

McCauley, J.F., 1973, Mariner 9 evidence for wind erosion in the
equatorial and mid-latitude regions of Mars: Journal of Geophysical
Research, v. 78, no. 20, p. 4123--4137.

\_\_\_\_\_\_\_\_1978, Geologic map of the Coprates quadrangle of Mars:
U.S. Geological Survey Miscellaneous Investigations Series Map I-897,
scale 1:5,000,000.

McCauley, J.F., Carr, M.H., Cutts, J.A., Hartmann, W.K., Masursky,
Harold, Milton, D.J., Sharp, R.P. and Wilhelms D.E., 1972, Preliminary
Mariner 9 report on the geology of Mars: Icarus, v. 17, no. 2, p.
289--327.

McGill, G.E., 1985, Age and origin of large martian polygons, *in*
Abstracts of papers submitted to the Lunar and Planetary Science
Conference, 16th, Houston, March 11--15, 1985, p. 534--535.

Moore, H.J., Arthur, D.W.G., and Schaber, G.G., 1978, Yield strengths of
flows on the Earth, Mars, and Moon: Lunar and Planetary Science
Conference, 9th, Houston, March 13--17, 1978, Proceedings, p.
3351--3378.

Morris, E.C., 1982, Aureole deposits of the Martian volcano Olympus
Mons: Journal of Geophysical Research, v.87, no. B2, p. 1164--1178.

Morris, E.C., and Dwornik, S.E., 1978, Geologic map of the Amazonis
quadrangle of Mars: U.S. Geological Survey Miscellaneous Investigations
Series Map I-1049, scale 1:5 000,000.

Morris, E.C., and Howard, K.A., 1981, Geologic map of the Diacria
quadrangle of Mars: U .S. Geological Survey Miscellaneous Investigations
Series Map I-1286, scale 1:5,000,000.

Mutch, T.A., Arvidson, R.E., Head, J.W., III, Jones, K.L., and Saunders,
R.S., 1976, The geology of Mars: Princeton, N. J., Princeton University
Press, 400 p.

Mutch, T.A., and Saunders, R.S., 1976, The geologic development of Mars:
A review: Space Science Reviews, v. 19, no. 1, p. 3--57.

Neukum, G.P.O., and Wise, D.U., 1976, Mars: A standard crater curve and
possible new time scale: Science, v.194, no. 4272, p. 1381--1387.

Pechmann, J.C., 1980, The origin of polygonal troughs on the northern
plains of Mars: Icarus, v. 42, no. 2, p. 185--210.

Peterson, Christine, 1981, A secondary origin for the central plateau of
Hebes Chasma: Lunar and Planetary Science Conference, 12th, Houston,
March 16--20, 1981, Proceedings, p. 1459--1471.

Phillips, R.J., Saunders, R.S., and Conel, J.E., 1973, Mars: Crustal
structure inferred from Bouguer gravity anomalies: Journal of
Geophysical Research, v. 78, no. 23, p. 4815--4820.

Pieri, D.C., 1980, Geomorphology of Martian valleys, *in* Advances in
Planetary Geology: National Aeronautics and Space Administration
Technical Memorandum 81979, p. 1--160.

Plescia, J.B., and Saunders, R.S.,1982, Tectonic history of the Tharsis
region, Mars: Journal of Geophysical Research, v.87, no. B12, p.
9775--9791.

Schaber, G.G., Horstman, K.C., and Dial, A.L., Jr., 1978, Lava flow
materials in the Tharsis region of Mars: Lunar and Planetary Science
Conference, 9th, Houston, March 13--17, 1978, Proceedings, p.
3433--3458.

Schonfeld, Ernest, 1979, Origin of Valles Marineris: Lunar and Planetary
Science Conference, 10th, Houston, March 19--23, 1979, Proceedings, p.
3031--3038.

Schultz, P.H., Schultz, R.A., and Rogers, John, 1982, The structure and
evolution of ancient impact basins on Mars: Journal of Geophysical
Research, v. 87, no. B12, p. 9803--9820.

Scott, D.H.,1979, Geologic problems in the northern plains of Mars:
Lunar and Planetary Science Conference, 10th, Houston, March 19--23,
1979, Proceedings, p. 3039--3054.

\_\_\_\_\_\_\_\_1982, Meander relics: Direct evidence of extensive
flooding on Mars, *in* Conference on Planetary Volatiles, Lunar and
Planetary Institute, Houston, p. 157--159.

Scott, D.H., and Carr, M.H., 1978, Geologic map of Mars: U.S. Geological
Survey Miscellaneous Investigations Series Map I-1083, scale
1:25,000,000.

Scott, D.H., and King, J.S., 1984, Ancient surfaces of Mars: The
basement complex, *in* Abstracts of papers submitted to the Lunar and
Planetary Science Conference, 15th, Houston, March 12--16, 1984, p.
736--737.

Scott, D.H., Schaber, G.G., Tanaka, K.L., Horstman, K.C., and Dial,
A.L., Jr., 1981, Map series showing lava-flow fronts in the Tharsis
region of Mars: U.S. Geological Survey Miscellaneous Investigations
Series Maps I-1266 to 1280, scale 1:2,000,000.

Scott, D.H., and Tanaka, K.L., 1980, Mars Tharsis region:
Volcanotectonic events in the stratigraphic record: Lunar and Planetary
Science Conference, 11th, Houston, March 17--21, 1980, Proceedings, p.
2403--2421.

\_\_\_\_\_\_\_\_1981, Mars: A large highland volcanic province revealed
by Viking images: Lunar and Planetary Science Conference, 12th, Houston,
March 16--20, 1981, Proceedings, p. 1449--1458.

\_\_\_\_\_\_\_\_1982, Ignimbrites of Amazonis Planitia region of Mars:
Journal of Geophysical Research, v. 87, no. B2, p. 1179--1190.

Sharp, R.P., 1973, Mars: Troughed terrain: Journal of Geophysical
Research, v. 78, no. 20, p. 4063--4072.

Sharp, R.P., and Malin, M.C., 1975, Channels on Mars: Geological Society
of America Bulletin, v. 86, no. 5, p. 593--609.

Siebert, Lee, 1984, Large volcanic debris avalanches: Characteristics of
source areas, deposits, and associated eruptions: Journal of Volcanology
and Geothermal Research, v. 22, no. 3/4, p. 163--197.

Sleep, N.H., and Phillips, R.J.,1979, An isostatic model for the Tharsis
province, Mars: Geophysical Research Letters, v.6, no. 10, p. 803--805.

Soderblom, L.A., Condit, C.D., West, R.A., Herman, B.M., and Kriedler,
T.J., 1974, Martian planetwide crater distributions: Implications for
geologic history and surface processes: Icarus, v. 22, no. 3, p.
239--263.

Soderblom, L.A., and Wenner, D.B., 1978, Possible fossil H20 liquid-ice
interfaces in the Martian crust: Icarus, v. 34, no. 3, p. 622--637.

Solomon, S.C., and Head, J.W.,1982, Evolution of the Tharsis province of
Mars: The importance of heterogeneous lithospheric thickness and
volcanic construction: Journal of Geophysical Research, v. 87, no. B12,
p. 9755--9774.

Tanaka, K.L., 1984, Probable lack of very ancient terrain on Mars
revealed by crater-population comparison with the Moon *in* Abstracts of
papers submitted to the Lunar and Planetary Science Conference, 15th,
Houston, March 12--16, 1984 p. 844--845.

\_\_\_\_\_\_\_\_1985, Ice-lubricated gravity spreading of the Olympus
Mons aureole deposits: Icarus, v. 62, no. 2, p. 191--206.

Thomas, P.C., and Veverka, Joseph,1979, Seasonal and secular variation
of wind streaks on Mars: An analysis of Mariner 9 and Viking data:
Journal of Geophysical Research, v. 84, no. B14, p. 8131--8146.

U.S. Geological Survey, 1976, Topographic map of Mars: U.S. Geological
Survey Miscellaneous Investigations Series Map I-961, scale
1:25,000,000.

Ward, A.W., 1979, Yardangs on Mars: Evidence of recent wind erosion:
Journal of Geophysical Research, v. 84, no. B14, p. 8147--8166.

Watters, T.R., and Maxwell, T.A., 1983, Crosscutting relations and
relative ages of ridges and faults in the Tharsis region of Mars:
Icarus, v. 56, no. 2, p. 278--298.

Wilhelms, D.E., and McCauley, J.F., 1971, Geologic map of the near side
of the Moon: U.S. Geological Survey Miscellaneous Geologic
Investigations Map I-703, scale 1:5,000,000.

Wilhelms,D.E., Oberbeck,V.R., and Aggarwal, H.R., 1978, Size-frequency
distributions of primary and secondary lunar impact craters: Lunar and
Planetary Science Conference, 9th, Houston, March 13--17, 1978,
Proceedings, p. 3735--3762.

Wilhelms, D.E., and Squyres, S.W.,1984, The martian hemispheric
dichotomy may be due to a giant impact: Nature, v.309, no. 5964, p.
138--140.

Willemann, R.J., and Turcotte, D.L., 1982, Role of lithospheric stress
in the support of the Tharsis rise: Journal of Geophysical Research, v.
87, no. B12, p. 9793--9801.

Wise, D.U., Golombek, M.P., and McGill, G.E., 1979, Tharsis province of
Mars: Geologic sequence, geometry, and a deformation mechanism: Icarus,
v. 38, no. 3, p. 456--472.

Witbeck, N.E., 1984, The geology of Mare Acidalium quadrangle, Mars,
*in* Advances in Planetary Geology: National Aeronautics and Space
Administration Technical Memorandum 86247, p. 219--419.

Woronow, Alexander, 1977, A size-frequency study of large Martian
craters: Journal of Geophysical Research, v.82, no.36, p. 5807--5820.

**NOTES ON BASE**

This map sheet is one of a series covering the entire surface of Mars at
a scale of 1:15,000,000. Sources for the map base were 1:5,000,000-scale
shaded relief maps described by Batson and others (1979). Data used in
the map portrayal were obtained from Viking Orbiter images.

ADOPTED FIGURE

The figure of Mars used for computing the map projections is an oblate
spheroid (flattening of 1/192) with an equatorial radius of 3,393.4 km
and a polar radius of 3,375.7 km.

PROJECTIONS

The Mercator projection is used between the 57° parallels; the Polar
Stereographic projection is used for the polar regions north and south
of the 55° parallels. Scales are 1:15,000,000 at the equator and
1:9,203,425 at the poles. The projections have a common scale of
1:8,418,000 at lat ±56°. Longitude increases to the west in accordance
with astronomical convention for Mars. Latitudes are areographic.

CONTROL

Planimetric control for the 1:5,000,000-scale maps used to compile the
bases for these sheets was derived from photogrammetric triangulations
by use of Mariner 9 pictures (Davies, 1973). This control net was
upgraded through the use of Viking data (Davies and others, 1978). At
least 85 percent of the image control points lie within 0.5 mm of the
positions published in 1978.

MAPPING TECHNIQUE

The mapping bases for this series were assembled from 1:5,000,000-scale
shaded relief maps reduced and digitally transformed where necessary to
fit the projections. During shaded relief portrayal, features on these
bases were used to position details taken from Viking Orbiter pictures.
Features were drawn as if illuminated uniformly from the west, through
use of airbrush techniques described by Inge (1972) and
photointerpretive methods described by Inge and Bridges (1976). The
shading is not generalized and accurately represents the character of
surface features.

Shaded relief analysis and portrayal were made by Barbara J. Hall.

NOMENCLATURE

All names on this sheet are approved by the International Astronomical
Union (IAU, 1974, 1977, 1980, 1983, and 1986). Named features and their
positions are taken from published maps of Mars that have scales of
1:2,000,000, 1:5,000,000, and 1:25,000,000.

M 15M 0/90 G Abbreviation for Mars; 1:15,000,000 series; center of map,
lat 0°, long 90°; geologic map, (G).

REFERENCES

Batson, R.M., Bridges, P.M., and Inge, J.L., 1979, Atlas of Mars, The
1:5,000,000 map series: National Aeronautics and Space Administration
Special Publication 438, 146 p.

Davies, M.E., 1973, Mariner 9: Primary control net: Photogrammetric
Engineering, v. 39, no. 12, p. 1297--1302.

Davies, M.E., Katayama, F.Y., and Roth, J.A., 1978, Control net of Mars:
February 1978: The Rand Corporation, R-2309-NASA, 91 p.

Inge, J.L. 1972, Principles of lunar illustration: Aeronautical Chart
and Information Center Reference Publication 72--1, 60 p.

Inge, J.L., and Bridges, P.M., 1976, Applied photointerpretation for
airbrush cartography: Photogrammetric Engineering and Remote Sensing, v.
42, no. 6, p. 749--760.

International Astronomical Union, 1974, Commission 16: Physical study of
planets and satellites, and Lunar *and* martian nomenclature, *in* 15th
General Assembly, Sydney, 1973, Proceedings: International Astronomical
Union Transactions, v. 15B, p. 105-108, 217-221.

\_\_\_\_\_\_\_\_1977, Working Group for Planetary System Nomenclature,
*in* 16th General Assembly, Grenoble, 1976, Proceedings: International
Astronomical Union Transactions, v. 16B, p. 321--325, 331--336,
355--362.

\_\_\_\_\_\_\_\_1980, Working Group for Planetary System Nomenclature,
*in* 17th General Assembly, Montreal, 1979, Proceedings: International
Astronomical Union Transactions v. 17B, p. 293--297.

\_\_\_\_\_\_\_\_1983 Working Group for Planetary System Nomenclature,
*in* 18th General Assembly, Patras, 1982, Proceedings: International
Astronomical Union Transactions, v. 18B, p. 334--336.

\_\_\_\_\_\_\_\_1986, Working Group for Planetary System Nomenclature,
*in* 19th General Assembly, New Delhi 1985, Proceedings: International
Astronomical Union Transactions, v. 19B, p. 347--350.