**GEOLOGIC MAP**

**OF THE**

**EASTERN EQUATORIAL REGION**

**OF MARS**

By

Ronald Greeley and J.E. Guest

1987

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--B, 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**

> Unit descriptions are based on characteristics observed primarily on
> Viking Orbiter images, supplemented by data from Mariner 9 and other
> remote-sensing instruments such as the Viking Infrared Thermal Mapper.
> Most units are grouped by geologic assemblages, according to the
> convention of Scott and Tanaka (1986).

**SURFICIAL DEPOSITS**

As SLIDE MATERIAL---Smooth to lobate flow material associated with some
scarps and crater rims; especially well developed along channel walls in
Deuteronilus Mensae region. Forms aprons along scarps and around knobs
and mesas; may completely cover channel floors. Primarily equivalent to
third type of slide material mapped by Scott and Tanaka (1986). No
specific type area. *Interpretation:* Unconsolidated material, such as
debris flows, resulting from mass wasting

Achu YOUNGER CHANNEL AND FLOOD-PLAIN MATERIAL, UNDIVIDED---In eastern
equatorial part of map area; forms plain as wide as 600 km marked by
dark, sinuous, intertwining albedo patterns; appears more mottled
westward. Type areas (Scott and Tanaka, 1986): lat 15° N., long 177°
(younger channel material) and lat 22° N., long 171° (flood-plain
material). *Interpretation:* Fluvial deposits; distinct albedo patterns
probably represent channels with bars and islands; mottled zones in
western part may represent deposition from ponded terminus of fluvial
system

Ad DUNE MATERIAL---Patches of barchan and other types of dunes on crater
floors. Makes up part of dunes and dune-capped material (unit **Ad**) of
south polar region (Tanaka and Scott, 1987). Type area: lat 48° S., long
329°. *Interpretation:* Sand deposited in areas of low wind strength

**LOWLAND TERRAIN MATERIALS**

> Consist of all plains-forming units between highland-lowland boundary
> scarp and north edge of map area, as well as materials of eastern
> volcanic assemblage.

**Northern plains assemblage**

> Materials deposited in widespread sheets on northern plains. Within
> each formation, members mapped on basis of morphology, albedo, and
> crater size-frequency distribution; some contacts approximately
> located. This assemblage postdates formation of highland-lowland
> boundary scarp (Scott, 1979).

Aps SMOOTH PLAINS MATERIAL---Forms patches and regions of flat,
featureless plains; lightly cratered. Type area: lat 9° N., long 222°.
*Interpretation:* Probably of diverse origin; many exposures probably
consist of eolian deposits

AHpe ETCHED PLAINS MATERIAL---Occurs as patches in Elysium Planitia.
Surface characterized by irregular mesas and pits. Type area: lat 15°
N., long 238°. *Interpretation:* Plains deposits mantled by eolian
material that has subsequently been eroded, possibly by wind

ARCADIA FORMATION---Forms low-lying plains (1) in Arcadia Planitia east
of Phlegra Montes in east margin of map area and (2) in Acidalia
Planitia as small patch in west margin. Common boundaries of older
members mapped arbitrarily in places. Flows with lobate margins, ridges,
and channels and small hills with summit craters visible in some areas.
Members 2 and 5 as defined by Scott and Tanaka (1986) not present in map
area. *Interpretation:* Mostly lava flows and small volcanoes

Aa4 Member 4---In Arcadia Planitia; overlies member 3. Type area: lat
45° N., long 175° (Scott and Tanaka, 1986)

Aa3 Member 3---In and north of Arcadia Planitia along east margin of map
area. Flow fronts visible in places. Type area: lat 15° N., long 155°
(Scott and Tanaka, 1986)

Aa1 Member 1---Near west and east borders of map area. Type area: lat
30° N., long 40° (Scott and Tanaka, 1986)

MEDUSAE FOSSAE FORMATION---Occurs near equator in eastern part of map
area. Consists of extensive, relatively flat sheets, generally smooth to
grooved and gently undulating; albedo moderate (Scott and Tanaka, 1986).
Locally, where upper or middle member stripped by wind, underlying
member shows lineations

Amu Upper member---Surfaces smooth, flat to rolling, light in color;
sculptured into ridges and grooves in places; broadly curved margins,
locally eroded into serrated scarps. Type area: lat 0° N., long 160°
(Scott and Tanaka, 1986). *Interpretation:* Thick deposits of eolian
sediments or volcanic pyroclastic deposits; wind eroded, particularly
along margins, to form yardangs

Amm Middle member---Similar to upper member but in places surface is
more rugged and eroded. Type area: lat 10° N., long 160° (Scott and
Tanaka, 1986). *Interpretation:* Poorly to moderately indurated eolian
or pyroclastic deposits; wind eroded, particularly along margins

Aml Lower member---Most widespread member in map area. Surfaces smooth
to rough and highly eroded, darker than those of other members. One area
centered at lat 1° S., long 182° contains long, broad troughs. Type
area: lat 0° N., long 174° (Scott and Tanaka, 1986). *Interpretation:*
Lava flows interbedded with eolian or pyroclastic deposits, in places
heavily eroded

VASTITAS BOREALIS FORMATION---Subpolar plains deposits of northern
lowlands; members distinguished on basis of morphology and albedo
contrast; placement of contacts locally arbitrary (Scott and Tanaka,
1986)

Hvm Mottled member---Major occurrence north of map boundary (Tanaka and
Scott, 1987); extends as far south as about lat 43° N. Crater-ejecta
blankets have higher albedo than adjacent terrain, giving mottled
appearance. In places, gently rolling, closely spaced hills averaging 5
km in diameter can be distinguished. Type area: lat 55° N., long 40°
(Scott and Tanaka, 1986). *Interpretation:* Possibly lava flows erupted
from fissures, or of alluvial or eolian origin

Hvg Grooved member---Occurs as isolated patches in several areas of
lowland plains; similar to mottled member but marked by curvilinear and
polygonal patterns of grooves and troughs; closed polygons as wide as 20
km. Ridges present in center of some grooves, as at lat 53° N., long
295°. Type area: lat 45° N., long 15° (Scott and Tanaka, 1986).
*Interpretation:* Material same as mottled member; patterns may be due
to compaction, tectonism, or periglacial processes

Hvr Ridged member---In isolated patches in northern plains.
Characterized by concentric, low ridges about 1 to 2 km wide; northwest
of Deuteronilus Mensae, many ridges are within depressions. In Isidis
basin, member displays low mounds, many of which are aligned. Type area:
lat 38° N., long 33° (Scott and Tanaka, 1986). *Interpretation:*
Material same as mottled member; unit appears to develop from erosion of
surrounding units. Origin of ridges unknown but they may result from
periglacial or erosional processes. Low mounds in Isidis basin may
represent spatter cones along lines of vents

Hvk Knobby member---Similar in appearance to mottled member but
generally has higher albedo and abundant small, dark, knoblike hills,
some with summit craters. Crater ejecta have albedo similar to that of
surrounding terrain. Type area: lat 55° N., long 5° (Scott and Tanaka,
1986). *Interpretation:* Plains of diverse origins (volcanic flows,
eolian mantles); hills may be small volcanoes, remnants of highland
terrain or of crater rims, or pingos

**Eastern volcanic assemblage**

> Volcanoes and lava flows in Elysium region (Greeley and Spudis, 1981).

ELYSIUM FORMATION---Units associated with Elysium Mons.

Ael4 Member 4---Channel material. Type area: lat 36.9° N., long 220°.
*Interpretation:* May be derived from lahars of members 1 and 3
(Christiansen and Greeley, 1981)

Ael3 Member 3---Forms plains having rugged relief, hummocky surfaces;
lobate deposits seen at high resolution. Type area: lat 43° N., long
230°. *Interpretation:* Of volcanic origin; flows possibly derived from
Elysium Mons, possibly interfinger with member 1. Extensively modified
by fluvial, eolian, and periglacial processes

Ael2 Member 2---Lobate deposits, with rilles, composing Elysium Mons
edifice. Gradational with member 1. Type area: lat 25° N., long 215°.
*Interpretation:* Lava flows displaying channels and partly collapsed
lava tubes

Ael1 Member 1---Lobate, plains-forming deposits that radiate from
Elysium Mons and overlie and embay Albor Tholus and Hecates Tholus
Formations. Type area: lat 32.5° N., long 214°. *Interpretation:*
Volcanic flows and related materials

AHat ALBOR THOLUS FORMATION---Material forming Albor Tholus; hummocky
texture, more subdued near vent. Type area: lat 18° N., long 210°.
*Interpretation:* Volcanic flows

Hhet HECATES THOLUS FORMATION---Forms Hecates Tholus; hummocky surface
cut by many narrow, sinuous channels. Type area: lat 33° N., long 209°.
*Interpretation:* Volcanic flows, some emplaced through lava channels

Hs SYRTIS MAJOR FORMATION---Plains-forming unit constituting Syrtis
Major Planum. Characterized by many lobate deposits radiating generally
from two irregular depressions (Nili Patera, centered at lat 9° N., long
293°, and Meroe Patera, centered at lat 7° N., long 291.5°); flow fronts
and margins clearly recognizable; mare-type ridges trend
north-northwest; many light and dark \"wind\" streaks on plains
deposits. Type area: lat 11° N., long 295°. *Interpretation:* Lava flows
erupted with low effective viscosity from central vents; rheological
properties similar to those of basaltic magmas

**HIGHLAND TERRAIN MATERIALS**

> Rock units of moderate to high relief; dominate southern and
> near-equatorial parts of map area.

**Hellas assemblage**

> Consists of units in and near Hellas Planitia.

Ah8 Knobby plains floor unit---Floor material characterized by low knobs
a few kilometers across. Type locality: lat 35° S., long 295°.
*Interpretation:* Possibly remnants of mantle that has been
differentially eroded; knobs may be small cinder cones

Ah7 Rugged floor unit---Forms undulating terrain of rugged relief on a
kilometer scale in western Hellas Planitia; one occurrence mapped. Type
area: lat 42° S., long 310°. *Interpretation:* Eroded mantle of possible
eolian origin overlying ridged plains material (unit **Hr**)

Ah6 Reticulate floor unit---Plains material characterized by reticulate
pattern of ridges; one occurrence mapped. Type area: lat 36° S., long
301.5°. *Interpretation:* Plains material whose high-standing remnants
result from differential erosion

Ah5 Channeled plains rim unit---Plains material on east rim of Hellas
basin characterized by narrow, sinuous channels and slight to moderate
relief; includes widespread

mesas having irregular margins and a few small knobs. Appears to fill
some craters. Type area: lat 43° S., long 264°. *Interpretation:* Mantle
of volcanic or eolian deposits eroded possibly by a combination of
fluvial and eolian processes. Channels may be fluvial or volcanic

Ah4 Lineated floor unit---Smooth plains material characterized by
straight and curvilinear lineaments; one occurrence mapped. Type area:
lat 34° S., long 295°. *Interpretation:* Possible mantle modified by
local tectonic processes

Hh3 Dissected floor unit---Smooth, rolling-plains material that has been
deeply dissected to produce local rugged relief. Occurs in central part
of Hellas Planitia. Type area: lat 40° S., long 289°. *Interpretation:*
Sedimentary deposits modified by wind and minor fluvial activity; may
include lava flows

Hh2 Ridged plains floor unit---Forms rolling smooth plains having
sinuous to linear mare-type (wrinkle) ridges; occupies outermost Hellas
Planitia as discontinuous concentric band. Type locality: lat 50° S.,
long 280°. *Interpretation:* Lava flows erupted with low effective
viscosity; composition possibly basaltic

Nh1 Basin-rim unit---Material of the Hellas basin rim. Rugged,
mountainous, heavily cratered, and modified by surficial processes, but
intermontane patches have little relief. Type locality: lat 52° S., long
261°. *Interpretation:* Impact-generated unit of ancient Martian crust;
consists of breccias and interbedded volcanic materials. Features
normally associated with basins, such as radial troughs, not seen,
probably covered by younger surficial deposits

**Plateau and high-plains assemblage**

> Forms ancient highland terrain and local tracts of younger deposits.

OLDER CHANNEL MATERIAL AND CHAOTIC MATERIAL

Hch Older channel material---Occurs mainly along boundary of northern
highlands of Deuteronilus Mensae but also in other highland locations,
including margin of Hellas basin. Channels generally steep sided, smooth
floored, and abruptly terminated on up-slope end. Type area: lat 25° N.,
long 60° (Scott and Tanaka, 1986). *Interpretation:* May be mixture of
channel deposits and mass-wasted materials; channels may have formed by
sapping

Hcht Chaotic material---Forms semicircular patches of closely spaced
knobs of similar heights; mapped near west map border just south of lat
40° N. and near east border. Type area: lat 5° S., long 27° (Scott and
Tanaka, 1986). *Interpretation:* Erosional remnants; not associated with
channels within map area

Nm MOUNTAINOUS MATERIAL---Forms large, very rugged, isolated blocks.
Scattered occurrences mostly around Hellas and Isidis basins. Equivalent
in part to mountain material (mapped as \"**m**\") in south polar region
(Tanaka and Scott, 1987). *Interpretation:* Mostly ancient crustal
material uplifted during formation of impact basins

PLATEAU SEQUENCE---Forms rough, hilly, heavily cratered to relatively
flat and smooth terrain covering most of highlands; occurs mostly in
southern hemisphere.

Hpl3 Smooth unit---Forms 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° (Scott and Tanaka,
1986). *Interpretation:* Interbedded lava flows and sedimentary deposits
of eolian or fluvial origin that bury most underlying rocks

Hplm Mottled smooth plains unit---Same as smooth unit but has mottled
albedo patterns. Global occurrences only in southwestern part of map
area. Type area: lat 38° S., long 358° *Interpretation:* Same as smooth
unit except that wind-sorted sediments have produced mottled appearance

Npl2 Subdued cratered unit---Forms highland plains characterized by
subdued and partly buried crater rims; fills some crater floors; flow
fronts rare. Type area: lat 28° S., long 162° (Scott and Tanaka, 1986).
*Interpretation:* Thin lava flows and sedimentary deposits that partly
bury underlying rocks

Npl1 Cratered unit---Widespread in southern highlands; highly cratered,
uneven surfaces of generally moderate, locally high relief; fractures
and channels common. Type area: lat 45° S., long 148° (Scott and Tanaka,
1986). *Interpretation:* Materials formed during period of high impact
flux; probably a mixture of volcanic materials, erosional products, and
impact breccia

Npld Dissected unit---Similar in occurrence and appearance to cratered
unit but more highly dissected by small channels, channel networks, and
troughs. Gradational with cratered unit; placement of contact based on
abundance of channels. Type area: lat 45° S., long 70° (Scott and
Tanaka, 1986). *Interpretation:* Origin same as that of cratered unit
but material more highly eroded by fluvial processes

Nple Etched unit---Similar to cratered unit but deeply furrowed by
sinuous, intersecting, curved to flat-bottomed grooves, producing an
etched or sculptured surface; commonly forms small mesas having
irregular margins; craters and other depressions filled with smooth
deposits. Occurs in several places on cratered plateaus, but is most
extensive and well developed northwest of Syrtis Major Planum. Type
area: lat 45° N., long 55° (Scott and Tanaka, 1986). *Interpretation:*
Cratered unit that has been partly mantled by deposits of possible
eolian origin and dissected by eolian erosion, decay and collapse of
ground ice, and minor fluvial activity

Nplr Ridged unit---Resembles and is locally gradational with 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° (Scott and Tanaka, 1986).
Interpretation Flood-lava flows; ridges due to faulting, folding, or
volcanic processes

Nplh Hilly unit---Rough, hilly, fractured material of moderately high
relief. Type area: lat 12° S., long 174° (Scott and Tanaka, 1986). Forms
complete or partial rims of Isidis and some other ancient basins.
*Interpretation:* Ancient highland rocks and impact breccia generated
during period of heavy bombardment

HNu UNDIVIDED MATERIAL---Forms closely spaced, conical hills a few
kilometers across whose distribution indicates that they are remnants of
numerous craters. Unit also forms rugged terrain on margins of cratered
plateaus and isolated remnants (as west of Orcus Patera near east map
border). Gradational with knobby plains material (unit **Apk**) where
units adjoin, but hills are more closely spaced, larger, and occupy more
than about 30 percent of area. No specific type area. *Interpretation:*
Most hills are eroded remnants of ancient cratered terrain produced by
mass-wasting processes, possibly as result of removal of ground ice;
some hills may be erosional remnants of intrusive bodies (Greeley and
Guest, 1978). Material may include some units of plateau sequence

HIGHLAND PATERAE

AHt Tyrrhena Patera Formation---Material forming Tyrrhena Patera; seen
on high-resolution images to consist of several members (Greeley and
Spudis, 1981), including a highly dissected basal member. Type locality:
lat 22° S., long 254°. *Interpretation:* Volcanic materials erupted
serially, including early-stage pyroclastic material and later lava
flows

AHa Apollinaris Patera Formation---Material composing Apollinaris
Patera. Consists of several members including deposits dissected by
channels, some of which flowed over a basal scarp; forms a large fanlike
feature on southeast flank. Type area: lat 9°

S., long 186°. *Interpretation:* Material of multiple-stage eruptions
forming shield volcano (Greeley and Spudis, 1981)

AHh Hadriaca Patera Formation---Material of Hadriaca Patera. Consists of
smooth floor material in central depression, locally ridged along
margin, surrounded by material dissected radially to depression. Type
area: lat 31° S., long 268°. *Interpretation:* Material of central-vent
volcano formed by multiple eruptions; possible pyroclastic deposits
indicate explosive activity (Greeley and Spudis, 1981)

Had Amphitrites Formation, dissected member---Forms shieldlike deposit
along north edges of Malea Planum (near south map border). Consists of
deeply etched, ridged, lobate deposits and superposed crater material;
sinuous furrows, ridges, and scarps trend toward Hellas Planitia. Type
area: northern Malea Planum. *Interpretation:* Ridged plains material
(unit **Hr**) modified by fluvial channeling

**MATERIALS OCCURRING THROUGHOUT MAP AREA**

Apk KNOBBY PLAINS MATERIAL---Extensive in northern lowlands where it
forms moderately to lightly cratered, generally smooth plains; several
isolated occurrences in cratered highlands. Conical hills or knobs occur
at irregular intervals; mare-type (wrinkle) ridges locally present.
Where adjacent to undivided material (unit **HNu**), units are
intergradational, but knobs in knobby plains unit are smaller and spaced
farther apart. Type area: lat 22° N., long 263°. *Interpretation:*
Probably of diverse origins but appears to have formed mainly by erosion
of older units. Knobs are probably erosional remnants but some may be
volcanic. Intervening plains may be erosional surfaces or may consist of
eolian, mass-wasted, or volcanic materials

Hr RIDGED PLAINS MATERIAL---Characterized by broad planar surfaces, rare
lobate deposits, and long, parallel, linear to sinuous mare-type
(wrinkle) ridges about 30 to 70 km apart. Forms plains within and
outside craters throughout plateau area and lowland plains north of
Orcus Patera (near east map border). Locally gradational with ridged
plateau material (unit **Nplr**) where units adjoin. Type area: Lunae
Planum, lat 10° N., long 65° (Scott and Tanaka, 1986). *Interpretation:*
Extensive lava flows erupted with low effective viscosity from many
sources at high rates; ridges either volcanic constructs or
compressional features

c , s IMPACT-CRATER MATERIALS---Yellow indicates materials of superposed
craters greater than about 100 km across; brown indicates materials of
partly buried craters greater than about 150 km across. May include rim
crest (hachured), central ring (inner circular feature, also hachured),
and central peak. Hachures also denote impact-basin rims. Symbol
\"**c**\" denotes crater-rim and ejecta material. Symbol \"**s**\" and
orange color denote smooth floor material; within mapped crater
material, only patches larger than 30 km across are shown; elsewhere,
only patches larger than about 80 km across are shown. Linear dot
pattern denotes secondary craters outside crater aprons.
*Interpretation:* Units resulting from impact cratering, but
smooth-floor material may be of volcanic, eolian, or fluvial origin

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

FAULT---Bar and ball on downthrown side

NARROW DEPRESSION---Fracture or graben

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

SCARP---Line marks top of slope; barb points downslope

NARROW SINUOUS CHANNEL---Wide age range

INFERRED FLOW DlRECTION OF FLOW LOBE

DEPRESSION OR CALDERA

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

VOLCANO---Age uncertain

**INTRODUCTION**

The Mariner 9 mission in the early 1970\'s provided the first
comprehensive view of the geology of Mars (McCauley and others, 1972;
Masursky, 1973) and led to the derivation of the first global geologic
map (Carr and others, 1973). These preliminary studies were followed by
more comprehensive geologic mapping coordinated by the U.S. Geological
Survey. Thirty quadrangles at a scale of 1:5,000,000 were produced in
this map series, from which Scott and Carr (1978) compiled a single
geologic map at a scale of 1 :25,000,000. The reliability of these maps,
however, is varied because of the uneven quality of the Mariner 9 data.

Viking Orbiter images fill important gaps in the Mariner data and
provide substantially better coverage at both moderate (300 to 130
m/pixel) and high (25 m/pixel) resolutions. The increased quality of the
images results from a better camera system, improved image processing,
and clearer atmospheric conditions than those of the Mariner 9 mission.
These factors, along with the higher resolution obtained by the Viking
Orbiter cameras, have enabled the discovery of geologic relations
previously unknown. Photogeologic results based on the Viking images,
together with results from other remote-sensing experiments such as the
Infrared Thermal Mapper flown on Viking Orbiter (Kieffer and others,
1977), provide the basis for compilation (at 1:15,000,000 scale) of the
three map sheets in the present series: the geologic map of the western
equatorial region of Mars (Scott and Tanaka, 1986), the geologic maps of
the polar regions of Mars (Tanaka and Scott, 1987), and this sheet.

Photogeologic mapping techniques employed here follow principles defined
by Shoemaker and Hackman (1962) for the Moon and refined by Wilhelms (in
press). Initial mapping was compiled on uncontrolled mosaics prepared
during Viking mission operations. Although these mosaics were produced
at a wide range of scales, were not uniform in format, and did not cover
the entire area of interest, this preliminary mapping provided insight
into local geologic relations. Results were then compiled on
1:2,000,000-scale photomosaics of moderate-resolution photographs (\~300
to 130 m/pixel). The final step involved recompilation from the
1:2,000,000-scale photomosaics onto the shaded relief 1:15,000,000-scale
map base (U.S. Geological Survey, 1982).

The map portrays inferred rock units, stratigraphic relations, and other
features such as faults. From this mapping, the major impact, tectonic,
and volcanic events that have shaped the surface of Mars are
interpreted, as are modifications produced by wind, water, and other
agents of gradation. Definition of possible rock units is based on
assessment of their geomorphology, albedo, spectral properties, and
thermal characteristics. As discussed by Milton (1974), some units on
Mars are characterized principally by their geomorphic expression, which
locally may be due more to secondary modification than to their primary
nature. Mapped units are assigned to time-stratigraphic systems
formulated from Mariner 9 mapping (Scott and Carr, 1978), which were
adapted for the map of the western equatorial region (Scott and Tanaka,
1986) and refined by Tanaka (1986).

Relative ages of the rock units are established by stratigraphic
relations, cross-cutting relations of structural features, and
size-frequency distributions of impact craters. Ages based on crater
frequency must be employed with caution, however, especially in areas
where wind and water have modified the surface and where sedimentary
deposits may have partly buried other units. As discussed by Condit
(1978), craters 4 to 10 km in diameter are important for use in broad
relative-age dating of Martian surfaces; smaller craters may have been
buried, eroded, or extensively modified, whereas larger craters are too
scarce to produce statistically meaningful results. Crater-density
numbers on the correlation chart were derived according to the
convention established by Scott and Tanaka (1986). Because small craters
are better preserved on the relatively smooth, younger geologic units
than on the rough, uneven, or highly faulted older materials, different
sizes of craters were counted for rock units in each system: greater
than 2 km for the Amazonian, greater than 2 and 5 km for the Hesperian,
and greater than 5 and 16 km for the Noachian.

Rock-stratigraphic classifications of different ranks (formations and
members) are used to minimize cumbersome adjectival descriptions. The
term "assemblage" (Scott and Tanaka, 1986) is employed for sets of units
that share common attributes; for example, the Hellas assemblage
encompasses those units associated with the formation and modification
of the Hellas basin. Many of the units appear to intergrade, and some
contacts are drawn primarily on the basis of secondary characteristics
such as discontinuities in impact-crater density; these contacts are
dashed.

Impact craters that are superposed on the surrounding terrain and have
well-preserved, extensive rim units (inferred to be ejecta deposits) are
shown in yellow; only those craters having material units larger than
100 km across are mapped. Craters whose various rim units appear to be
buried or partly buried are shown in brown; only those craters having
rim diameters larger than 150 km are mapped. Craters are not otherwise
classified by type or age.

**PHYSIOGRAPHIC SETTING**

Mars\' surface can be divided into two major physiographic provinces,
the northern lowlands (which include the north polar region) and the
southern cratered highlands. The boundary between the two provinces
forms an approximate great circle inclined to the equator by about 28°.
Both provinces are exposed in the map area, although the cratered
highlands are the prevailing terrain. The boundary between the two
provinces in the western part of the map area is marked by a scarp as
high as several kilometers. The terrain on the upland side of this scarp
displays grabens and fractures, many of which may have been modified by
channels that are presumably of fluvial or sapping origin. This surface
was termed "fretted terrain" by Sharp (1973). Valleys between
high-standing remnants of uplands in the fretted terrain are floored by
channel deposits, debris aprons, and sediments inferred to be eolian.
The upland-lowland boundary in the eastern part of the map area is more
gradational and lacks a conspicuous scarp and fracture system. In the
middle of the map area, the boundary is embayed from the northeast by
younger lowland plains that fill what is probably a degraded impact
basin, Isidis.

Although materials of the northern lowlands are generally younger than
those of the southern cratered uplands, younger intercrater plains
material also occurs in the southern province. These deposits appear to
be diverse and may include eolian and fluvial sediments as well as
volcanic materials (Greeley and Spudis, 1978). The dominant feature in
the southern province is the Hellas basin interpreted as an impact scar,
whose rim diameter is about 1,800 km.

The Elysium region, one of the major volcanic provinces on Mars, lies
within the northern lowlands. Containing several volcanoes and multiple
lava flows derived from central vents and probably from fissures, the
Elysium region appears to be a smaller and generally older version of
the Tharsis volcanic province in the western hemisphere.

Large impact basins are not as obvious on Mars as on the Moon and
Mercury. Although their scarcity may represent a difference in impact
flux, a more likely explanation is that erosion and resurfacing have
obliterated much of the early Martian crustal record. Features
representing the remnants of possible basins other than Hellas and
Isidis (such as Aram Chaos and the "Ladon basin") have been described by
Wood and Head (1976) and Schultz and others (1982).

**STRATIGRAPHY AND HISTORY**

**Noachian System**

The Noachian System comprises the oldest rocks discernible on Mars
(Scott and Carr, 1978). Size frequencies of craters larger than 16 km in
diameter on Noachian units correlate with those of the Nectarian System
on the Moon and may represent a similar period of impact cratering
(Tanaka, 1986).

Two of the oldest rock units on Mars are *basin-rim material* (unit
**Nh1**) and *hilly* material (unit **Nplh**), which form rugged,
mountainous terrain associated with the Hellas and Isidis basins,
respectively; a third ancient unit, *mountainous material* (unit
**Nm**), forms isolated patches of more rugged terrain. This third unit
does not everywhere appear to be associated with a clearly recognizable
basin, but it is considered to represent parts of basins now mostly
destroyed.

Overlying these rugged and intensely cratered terrains are old plateau
units, including *cratered material* (unit **Npl1**) and *subdued crater
material* (unit **Npl2**) that may in part represent early-stage,
flood-style volcanism (Greeley and Spudis, 1981); such a volcanic origin
is indicated by the presence of large mare-type (wrinkle) ridges in the
*ridged material* (unit **Nplr**). The *dissected material* (unit
**Npld**) has been cut by extensive channels that are presumably of
fluvial origin; the etched material (unit **Nple**) appears to be
mantled by sediments of possible eolian origin that, in turn, have been
partly eroded to produce an etched appearance.

*Undivided material* (unit **HNu**) consists of rock units in the
highlands and lowlands that cannot be distinguished morphologically or
placed precisely in a stratigraphic position in the Hesperian or
Noachian Systems.

Rock units of Noachian age are considered to result from the final
stages of planetary accretion and the waning period of heavy
bombardment. In the middle to late Noachian Period, we infer the
extrusion of extensive flood lavas and contemporaneous surface
modifications by eolian, fluvial, and other surface processes that
continued into the Hesperian Period.

**Hesperian System**

As in the western hemisphere (Scott and Tanaka, 1986), many highland
areas are interpreted to have been smoothed and resurfaced by lava flows
and eolian deposits, representing a continuation of activity that began
during the Noachian. Units of Hesperian age are distinguished by having
67 to 200 craters larger than 5 km in diameter per 106 km2.

The most extensive plains unit in the Hesperian System is the *ridged
plains material* (unit **Hr**), which has sinuous ridges resembling
those of the lunar maria and is inferred to be lava flows. It forms the
basal unit of the Hesperian System (Scott and Carr, 1978). The largest
concentration of ridged plains material in the map area is in Hesperia
Planum northeast of the Hellas basin. In Syrtis Major Planum, a
widespread occurrence of apparently younger material is designated the
*Syrtis Major Formation* (unit **Hs**); unlike the ridged plains
material (unit **Hr**), flow units of the Syrtis Major Formation have
clearly distinguishable flow fronts and margins, and they appear to have
originated from at least two central features resembling calderas
(Schaber, 1982). Although these flows display many ridges, the ridges
are generally narrower and shorter than those of the ridged plains
northeast of Hellas.

In addition to eruptions in Syrtis Major, the middle Hesperian Period
was marked by the initiation of central-vent volcanism in the Elysium
region: lavas were extruded to form the *Hecates Tholus Formation* (unit
**Hhet**) and the older flows of the *Albor Tholus Formation* (unit
**AHat**). Eruptions also occurred in the southern hemisphere to form
the *dissected member* of the *Amphitrites Formation* (unit **Had**) and
most of the *Tyrrhena Patera Formation* (unit **AHt**), the *Apollinaris
Patera Formation* (unit **AHa**), and the *Hadriaca Patera Formation*
(unit **AHh**). These eruptions may be associated with crustal
adjustments following the formation of the Hellas basin (Peterson,
1978). On the basis of photogeologic analysis, Greeley and Spudis (1981)
postulated that Tyrrhena Patera was formed by magmas erupted through
impact-generated regolith that contained ground water. This process may
have resulted in phreatomagmatic explosions that produced an early-stage
ash shield, which was later degraded by surficial processes and which
was the site of late-stage effusion of lavas. Although images of
Amphitrites (lat 61° S., long 299°), Apollinaris, and Hadriaca Paterae
are inadequate for detailed analysis, a similar history is proposed for
these eruptive centers

Further resurfacing of the southern highlands is represented by the
youngest units of the plateau sequence, which subdue the underlying
terrain. These units are characterized by smooth surfaces and are
separated on the basis of albedo patterns into *smooth material* (unit
**Hpl3**) and *mottled plains material* (unit **Hplm**). Both units may
consist of interbedded lava flows and sedimentary deposits; the mottling
of the mottled plains unit may be caused by wind-sorted sediments.

The *Hellas assemblage* consists of units in and around Hellas Planitia.
Its lowermost unit, the Noachian basin-rim material noted above, is
overlain by the *ridged plains floor unit* (unit **Hh2**, presumed to be
flood lavas contemporaneous with the ridged plains unit), which is in
turn overlain by the *dissected floor unit* (unit **Hh3**) that may
consist of modified eolian, fluvial, and lava-flow materials.

*Older channel material* (unit **Hch**) was also emplaced during the
Hesperian along the northern lowlands-southern highlands boundary and on
the northeast margin of the Hellas basin. Channel formation along the
boundary was concentrated near Deuteronilus Mensae and represents major
modification of the scarp boundary. East of Deuteronilus, channels are
not abundant; however, scarp modification or retreat is indicated by
knobby material mapped as part of the undivided unit (unit **HNu**).
Parts of the southern highlands were modified by erosion to produce
*chaotic material* (unit **Hcht**).

The *Vastitas Borealis Formation* (Scott and Tanaka, 1986) occurs in the
northern lowlands as part of the northern plains assemblage. It consists
of four members representing various stages of degradation and
modification: *knobby member* (unit **Hvk**), *mottled member* (unit
**Hvm**), *grooved member* (unit **Hvg**), and *ridged member* (unit
**Hvr**). The oldest and most widespread, the knobby member, is
characterized by many closely spaced, conical hills that appear darker
than the surrounding terrain. The mottled member forms an extensive zone
that nearly encircles the planet between about lat 50° and 70° N.;
although Mariner 9 images indicate only a blurred surface having high
albedo contrast, Viking images show that the contrasting albedo is due
to impact craters whose ejecta deposits are brighter than intercrater
areas (Witbeck, 1984, p. 296). The grooved member is characterized by
irregular to polygonal patterns formed by grooves and troughs as long as
20 km, which may have been produced by periglacial, tectonic, or
compaction processes (Carr and Schaber, 1977; Pechmann, 1980; McGill,
1985) enhanced by erosion. The ridged member is distinguished by
whorl-like patterns of ridges resembling fingerprints. Stratigraphic
relations between the Vastitas Borealis Formation and the younger
Arcadia Formation are unclear except in eastern Acidalia Planitia, where
the lowermost member (unit **Aa1**) of the Arcadia Formation overlies
the knobby member of the Vastitas Borealis Formation.

Contemporaneous with formation of the younger Vastitas Borealis members
was the beginning of the development of the *etched plains material*
(unit **AHpe**) in southern and western Elysium Planitia. This unit is
characterized by irregular mesas and pits representing the erosion of a
mantle, possibly by the wind.

**Amazonian System**

This system was defined by Scott and Carr (1978) to include the youngest
units on Mars, most of which occur in the northern lowlands. As seen on
Mariner 9 images, the units generally form sparsely cratered,
featureless plains. However, Viking data reveal more complex terrains
and suggest diverse origins and modification by a variety of processes.
Amazonian units are subdivided by age on the basis of frequency
distributions of craters larger than 2 km in diameter.

In addition to the northern plains units, the Amazonian System includes
the Elysium Formation and materials formed during the modification of
the Hellas basin. During the early part of the Amazonian, central-vent
volcanism continued at Albor Tholus. Its flows, as well as those of the
Hecates Tholus Formation, are overlain by plains-forming lava flows of
*member 1* (unit **Ael1**) of the *Elysium Formation*. These
fresh-appearing flows are radial to the Elysium Mons edifice and are
gradational with flows of *member 2* (unit Ael2) that form the edifice;
these younger flows also appear fresh and display channels and features
interpreted as collapsed lava tubes. Northwest of Elysium Mons, in
Utopia Planitia, *member 3* (unit **Ael3**) forms rugged plains having
hummocky surfaces and flow lobes; the member appears to interfinger with
member 1. The plains have been extensively modified by processes
inferred to be fluvial, eolian, and periglacial. Part of this
modification may be related to channels that also cut member 1. The
channel material, mapped as *member 4* (unit **Ael4**), is considered
volcanic, possibly derived from lahars of Elysium Mons (Christiansen and
Greeley, 1981).

Partial filling, mantling, and modification of the Hellas basin
continued through the middle of the Amazonian Period to produce the
younger units of the Hellas assemblage. Most of these units occur near
the basin rim and are considered to be mantling material, some of which
is eroded. The *lineated floor unit* (unit **Ah4**) and the *channeled
plains rim unit* (unit **Ah5**) have similar crater-count ages and may
represent only slightly modified areas of a mantle. The lineated floor
unit is characterized by straight and curvilinear lineaments within
smooth plains material and may have formed by tectonic modification of
mantling material. The channeled plains rim unit has a subdued
appearance and low remnant mesas and narrow channels. The other younger
Hellas units form plains distinguished by low-relief features that may
reflect differential erosion of mantling material. These units are
*reticulate floor material* (unit **Ah6**) characterized by a reticulate
pattern of ridges, a *rugged floor unit* (unit **Ah7**) whose undulating
surface is rugged on a kilometer scale, and a *knobby plains floor unit*
(unit **Ah8**) that forms low knobs a few kilometers across.

In the northern plains assemblage, the *Arcadia Formation* is inferred
to consist of lava flows whose age range defines and spans the Amazonian
Period (Scott and Carr, 1978). Of the five members present in the
western equatorial region (Scott and Tanaka, 1986), only three occur in
the eastern region: *member 1* (unit **Aa1**), *member 3* (unit
**Aa3**), and *member 4* (unit **Aa4**). Lobate flow fronts, crescentic
lava-flow ridges, and features presumed to be lava channels are visible
on high-resolution images and are considered to be evidence of volcanic
origin.

The *Medusae Fossae Formation* (Scott and Tanaka, 1986) consists of
three members distributed along an east-west border zone between the
highland plateau and lowland plains of Amazonis Planitia in the eastern
part of the map area. The *lower*, *middle*, and *upper members* (units
**Aml**, **Amm**, and **Amu**, respectively) are distinguished on the
basis of morphology, stratigraphic relations, and albedo. The materials
are seen on high-resolution Viking images as massive horizontal sheets
that may have a combined thickness in excess of 3 km. They have smooth
and flat to gently rolling surfaces that are lineated in places where an
upper member has been stripped by wind. Eolian erosion has also produced
yardangs on the edges of some high-standing deposits of the upper
member, which suggests that they are composed of friable materials
(Ward, 1979). The deposits are less hilly and cratered than those of the
highlands, but they have higher relief and a higher albedo than the
plains materials . Although Scott and Tanaka (1982) preferred a volcanic
origin for this formation, suggesting that it consists of ignimbrites
and lava flows, it may be formed of vast accumulations of eolian
deposits transported and trapped along the highland-lowland boundary
(Lee and others, 1982; Thomas, 1982) or of materials related to ancient
polar deposits (Schultz and Lutz-Garihan, 1981).

Resurfacing or modification of parts of the northern lowlands continued
through the Amazonian Period to produce plains materials of diverse
origins. These materials have been divided into a featureless *smooth
plains unit* (unit **Aps**) and a *knobby plains unit* (unit **Apk**).
In many places the smooth plains unit probably consists of eolian
deposits. The knobby plains unit appears to be formed by the erosion of
older units; surfaces between the knobs may be erosional or may consist
of volcanic, mass-wasting, or eolian materials.

Surficial materials of inferred mass-wasting, fluvial, and eolian origin
occur locally throughout the eastern hemisphere of Mars. However, in
only a few places are deposits extensive or continuous enough to be
mapped as separate units. *Slide material* (unit **As**) forms aprons
around knobs and fills channels in the fretted terrain of Deuteronilus
Mensae and eastward. This material, interpreted to result from mass
wasting, clearly overlies both the older channel material

(unit **Hch**) and crater ejecta on the channel floors. *Younger channel
and flood-plain material, undivided* (unit **Achu**), located south of
the Elysium region, is characterized by albedo patterns that appear to
represent channels and bars formed by fluid flow. The paucity of
superposed craters on this material indicates that channel formation may
have occurred late in the Amazonian Period. Eolian processes also were
significant during Amazonian time. In addition to the eolian deposits
that mantle underlying terrain and probably make up at least part of the
smooth plains material of the northern plains assemblage, evidence of
eolian activity includes *dune material* (unit **Ad**) and wind streaks
that form albedo patterns in the lee of obstacles. In some areas the
streaks are dark and may represent patches of older rocks exposed by
erosion.

**Unassigned Materials**

Mantling deposits form smooth plains on the floors of many craters. In
some places the materials may consist of lava flows extruded from
fractures in crater floors (as on the Moon and probably on Mercury); in
other areas they may be eolian and alluvial deposits. The mantling
deposits are shown in orange and designated by the symbol \"**s**\".

One mountain, interpreted as volcanic, straddles the south boundary of
the map area at long 220°-224°.

**STRUCTURE**

The major tectonic features on Mars fall into four broad categories: (a)
the northern lowlands-southern highlands boundary, (b) the Tharsis and
Elysium rises, (c) Valles Marineris, and (d) crustal deformation
associated with large impact structures. Of these features, part of the
northern lowlands-southern highlands boundary, the Elysium rise, and
some large-impact crustal deformation occur within the map area. Several
hypotheses have been proposed to explain the northern-southern
dichotomy, including origins related to global expansion (Mutch and
others, 1976, p. 233), thinning of the lithosphere due to first-order
mantle convection early in Martian history (Wise and others, 1979), and
impact-basin formation (Wilhelms and Squyres, 1984). However, compelling
evidence to support or refute these ideas has not been advanced and the
problem remains enigmatic.

Gravity data (Sjogren, 1979) reveal large positive anomalies over the
Tharsis rise and the Elysium volcanic field. Several interpretations
have been offered to explain these anomalies. For example, Solomon and
Head (1979, 1982) proposed a model that involves crustal thickening by
the accumulation of lava flows and other volcanic products on a
lithosphere whose thickness has varied with location and time. This
model is consistent with analyses of concentric fractures surrounding
Elysium Mons generated by mass loading (Comer and others, 1985) and with
the gravity data (Janle and Ropers, 1983). Although applied principally
to the Tharsis rise, the model may apply also to the Elysium region
(Solomon and Head, 1982; Hall and others, 1983).

Basin-forming impacts have had major effects on planetary surfaces. They
have deposited ejecta that may mantle thousands of square kilometers,
and they have generated seismic waves that may have been felt
planet-wide. They have also extensively fractured the lithosphere: the
structural imprint of a basin may be imposed over a large area.
Adjustments of the crust and subsequent volcanism also are common
postimpact effects. As noted above, many of the volcanic paterae on Mars
may have formed over ring fractures around the Hellas basin (Peterson,
1978). Similarly, the eruption of the Syrtis Major flows may represent
volcanism associated with the Isidis basin, as the probable source vents
are positioned over extrapolated ring fractures around the basin (Comer
and others, 1985).

**ACKNOWLEDGMENTS**

This mapping project has extended over several years. During this time,
we have benefited from numerous discussions with our colleagues
regarding Martian geology, and we especially acknowledge M.H. Carr, D.H.
Scott, K.L. Tanaka, and D.E. Wilhelms. Mapping was aided greatly by
reviews by E.H. Christiansen, E.F Albin, Paul Spudis, and David Pearse.
We are particularly grateful to Eileen Theilig for her contributions and
untiring good spirits in several recompilations of various versions of
the map. This work was supported by the Planetary Geology and Geophysics
Program of the National Aeronautics and Space Administration through
Grant NSG-7548 to Arizona State University.

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Greeley, Ronald, and Guest, J.E., 1978, Geologic map of the Casius
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Janle, Peter, and Ropers, Jan, 1983, Investigation of the end isostatic
state of the Elysium dome on Mars by gravity models: Physics of the
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Lee, S.W., Thomas, P.C., and Veverka, Joseph, 1982, Wind streaks in
Tharsis and Elysium: Implications for sediment transport by slope winds:
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Masursky, Harold, 1973, An overview of geological results from Mariner
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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
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**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 15M0/270G Abbreviation for Mars l:15,000,000 series; center of map,
lat 0°, long 270°; geologic map, (G).

REFERENCES

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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.

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