Global geologic mapping of Mars: The western equatorial region

Ada. Space Rca. Vol.5, No.8, Printed in Great Britain.

pp.li—82, 1985

0213~1177/85 $0.00

+ .50 COSPAR

GLOBAL GEOLOGIC MAPPING OF MARS: THE WESTERN EQUATORIAL REGION David H. Scott U.S. Geological Survey, Branch of Astrogeology, 2255 North Gemini Drive, Flagstaff, AZ 86001, U.S.A.

ABSTRACT Global geologic mapping of Mars was originally accomplished following acquisition of orbital spacecraft images from the Mariner 9 mission. The mapping program represented a joint enterprise by the U.S. Geological Survey and other planetary scientists from universities in the United States and Europe. Many of the Mariner photographs had low resolution or poor albedo contrast caused by atmospheric haze and high—sun angles. Some of the early geologic maps reflect these deficiencies in their poor discrimination and subdivision of rock units. New geologic maps made from higher resolution and better quality Viking images also represent a cooperative effort, by geologists from the U.S. Geological Survey, Arizona State University, and the University of London. This second series of global maps consists of three parts: 1) western equatorial region, 2) eastern equatorial region, and 3) north and south polar regions. These maps, at 1:15 million scale, show more than 60 individual rock—stratigraphic units assigned to three Martian time—stratigraphic systems. The first completed map of the series covers the western equatorial region of Mars. Accompanying the map is a description of the sequence and distribution of major tectonic, volcanic, and fluvial episodes as recorded in the stratigraphic record. INTRODUCTION The validity and scope of planetary photogeologic mapping depend upon the quality, resolution, and areal coverage of spacecraft images. The reliability of geologic interpretations made from photographs of the planets is related to the experience of the observer in recognizing features that appear to have terrestrial analogs. Mars, the most Earth—like planet yet studied in the Solar System, exhibits a variety of features having terrestrial counterparts. For this reason its geology is more readily understood and appreciated than that of other planets, particularly those of the outer Solar System. (However, Earth’s geologic variety appears to surpass them by far: if our habitat had been Mars or any planet other than Earth, our base of experience would be much more restricted. We would have great difficulty in understanding the diversity of terrestrial geologic features). Preliminary geologic studies of Mars made from Mariner 9 images show a surface that not only reflects a record of intense meteorite impacts like that of the Moon, but one that also has been shaped by wind, water, ice, volcanic eruptions, and tectonic activity. The first systematic geologic mapping of the planet began as a cooperative project of the U.S. Geological Survey, a number of universities both in the United States and Europe, and a few research centers. A series of 30 geologic maps of Mars at 1:5.-million scale was produced that culminated in a global geologic map of Mars at 1:25—million scale /1/. This map has provided a continuing and useful reference base for a variety of Martian research projects. Images of Mars obtained from Viking Orbiters 1 and 2 are far superior to those of Mariner 9 in clarity, resolution, and amount and detail of coverage. These pictures promoted a renewed interest in photogeologic studies of Mars and fostered another joint mapping venture designed to supersede previous maps made from Mariner 9 pictures. This new series of global maps is at 1:15-million scale will be published in three parts: 1) western equatorial region, 2) eastern equatorial region, and 3) north and south polar regions. The western and polar regions are being mapped by geologists of the U.S. Geological Survey, the eastern region by geologists from Arizona State University and the University of London. The new maps will show more than 60 individual rock—stratigraphic units assigned to the three Martian time—stratigraphic systems formally defined by the Mariner 71

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geologic mapping /1/. This paper is a discussion of the western equatorial geologic map of Mars, the first to be completed in the series. The map shows the distribution and sequence of major tectonic, volcanic, and fluvial episodes that have contributed to the evolutionary history of the planet. Geologic units were identified and mapped from 1:2 million—scale photomosaics and individual photographs at moderate to high (300—130 rn/pixel) resolution. The description and classification of rock—stratigraphic units was based on inferred lithologic characteristics recognized on high— to very high resolution pictures (130—15 m/pixel). Relative ages of the rock units were established by stratigraphic and structural relations, morphologic appearance, and crater-density studies. The map units were broadly divided into two major physiographic provinces, depending on their occurrence in either highland or lowland regions. The lowland terrain consists of extensive, relatively smooth plains in the northern hemisphere that are separated from the more rugged highlands to the south by an irregular scarp extending almost completely around the planet. The generally featureless lowland plains are covered in places by small knobs and conical hills that may be of volcanic origin /2/. Highland terrain consists of rock and rock—tectonic units of medium to high relief that dominate the southern and equatorial regions. Rockstratigraphic classifications, used for the first time in the geologic mapping of Mars, include formations and their members. Some units appear to be intergradational; where their boundaries are indistinct they are mapped with the aid of crater-density distributions. In order to minimize excessive detail, only craters larger than 150 km are mapped. The Viking map series is affected by some of the uncertainties inherent in the earlier maps of Mars, which resulted from the modification and obscuration of primary characteristics of rocks by erosion and tectonic overprint. For example, deformation has destroyed all identifying lithologic characteristics of the terrain in the highly faulted and fractured areas north and south of Tharsis Montes and in the Tempe Terra plateau. Some of the more important changes included on the new map of the western equatorial region are the subdivision of lava flows around large volcanic centers, the extension of channel and floodplain deposits far into the northern plains, the mapping of large areas of ignimbrite deposits and the discovery and mapping of another volcanic province in the highlands south of Tharsis Montes /3/. In addition, the number of map units has been more than doubled, and several rock units have been reassigned to different time—stratigraphic systems. GEOLOGIC SUMMARY A topographic rise of regional extent dominates the western equatorial region of Mars. It is centered around Tharsis Montes—Syria Planum and includes the four largest and youngest volcanoes on Mars (Figure 1). Volcanic materials constitute the primary rock types on Mars, particularly in the western equatorial region. Lava flows are similar in morphology to those on the Earth and Moon. They comonly exhibit oven appi ng, 1 obate, and crenul ated margins and form long narrow tongues, sheet flows, or channel- and tube-fed flows. Sheet flows are common on the plains and on the lower, more gentle slopes of volcanoes. High— resolution pictures show concentric ridge patterns subparallel to flow margins (Figure 2). Channel and tube flows comonly associated with long, narrow flows are prevalent on steeper slopes of volcanoes. Younger lava flows have rougher textures than older ones that have been smoothed by erosion and mantled by eolian deposits. Martian lava flows, like those on Earth, originated from central volcanic vents, radial fissures on volcano flanks, or fissures in plains areas far removed from volcanic edifices. The more important geologic units are discussed in the following sections. STRATIGRAPHY Noachian System The Noachian System consists of the oldest rock units on Mars. They are believed to be correlative in age with those of the Imbrian and Nectanian Systems on the Moon /4/. The basement complex (Figure 3a, b) underlies the oldest identifiable rocks in the western hemisphere of Mars /5/; It consists of highly faulted, fractured, and cratered surfaces projecting above relatively smooth, lava—covered terrain. The composition of the basement rocks is unknown because of their structural deformation; however, they are rugged, have prominent relief, and probably consist of highly resistant materials. In places, small islands of the basement (Figure 3b) resemble erosional remnants of other older topography that has been partly buried by more recent lava flows. Basement rocks are transitional, in places, with other complexly faulted material whose surfaces are smoother and have less relief. The origin and composition of these rocks are also unknown, except that they predate major teconic episodes that uplifted and faulted the Tharsis-Syria rise /6/. All of these basal rock units of the Noachian System may consist mostly of impact breccias formed during the early stages of high meteorite bombardment, similar to that of the lunar highlands.

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Fig. 1. Index map of western equatorial reyicn of Mars showing nalor physioqro~hic features. Numbered rectangles indicate locations of figures 2 trr...igh 10.

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Fig. 3a. Basement complex (Nb) embayed by lower member of Syria Planum Formation (Hsl). Highly fractured material (Nf) embayed on east by upper member of Syria Fianur;i Formation (Hsu) and embayed on west by Tharsis Montes lava flows :At.~). kelat’’.ely young volcano (V) superposed on fractured material. Picture is ahou~900 I~ wi~e;

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Fig. 3b. Small (i.15 km diameter) “islands” of basement (Nb) project above cratered plateau unit (Nplc). Note that grabens transecting cratered plateau unit are :..ried by channel deposits (Nch) which, in turn, are partly overlapped by mottled plalrs member of Vastitas Borealis Formation (Avm).

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Cratered plateau material (Figure 4) is the most extensive rock unit in the Noachian

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Fig. 4. Cratered plateau material (Nplc) overlapped In places by ridged plains unit (Hpr) and postulated ignimbrites (Am). Arrows indicate locations of remnant mesas of ignimbrites shown In Figure 11. System. As its name implies, the unit is distinguished by a dense crater population and rough, low—relief lntercrater areas. Cratered plateau material is the most widespread rock unit of the highlands, and its surface has been highly modified by post—accretionary meteorite impacts. It probably consists of lava flows and interbedded breccias that were distributed as ejecta blankets around numerous craters. The cratered plateau material has been divided into subunits where it is traversed by prominent ridges or where it has been dissected and etched by wind, water, or the collapse of ground ice. Hesperian System Many areas within the highly cratered highlands have been smoothed and subdued by subsequent lava flows and eolian deposits that have not completely obscured their ancient origin. Materials forming this terrain are assigned to the Hesperlan System. The most extensive lava flows within the Hesperian System are the ridged plains material (Figure 5). It covers the long, broad plateau extending from Lunae Planum southward beyond Valles Marineris. It also occurs in scattered patches elsewhere In the highlands and the lowland plains. The flows closely resemble lava flows on the lunar maria: they exhibit long, sinuous wrinkle ridges and smooth inter—ridge surfaces that are much less cratered than the highlands. The ridged plains material forms the basal rock—stratigraphic unit of the Hespenian System /1/. The Syria Planum Formation (Figure 3a) and the lower members of the Tharsis Montes (Figure 6) and Alba Patera Formations were also deposited during the Hesperian Era, and are closely equivalent in age. These lava flows are all associated with central volcanic edifices but some were extruded from fissures In the volcanoes’ flanks. Older flows from Alba Patera nearly encircle the volcano out to a distance of about 1800 km, and they also ecnbay the highly fractured terrain that occurs In a broad arc to the north of the Olympus Mons aureoles. Materials forming the walls and parts of the floor of Valles Marineris and the systems of channels leading into Chryse Basin are mostly Hesperlan in age. Crater densities within the channels and floodplains vary from place to place, suggesting that water flooding occurred episodically over a long period of time. Relict cresentic depressions that in

size and shape resemble meander patterns of terrestrial rivers (Figure 7) are recognized deep within the lowland plains of Chryse Basin. They are partly buried by the Arcadia J~st ~8-~

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Fig. 5. Ridged plains material (Hpr) partly overlapping large “island of fractured rocks (Nf) in lower part of picture. Ridge structures here are older than superposed impact craters and cross—cutting fault systems.

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Fig. 6. Lower member of Tharsis Montes Formation (Ht ) overlaps ridged plains unit (Hpr) along line indicated by arrows; older fault sys~emsin ridged plains are buried by younger lava flows. In this area ridge structures are sparse.

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Fig. 7. Chryse Basin area. Arrows indicate river channels that are partly buried by lava flows and/or eolian deposits in northern plains. Central ridges are characteristic of many other channels on Mars. Formation of Amazonian age but can be recognized by narrow central ridges that are characteristic of channel troughs elsewhere in the river systems. They occur as far north as lat 450 N., indicating that early periods of water flooding were extensive in the northern plains. Evidence of alluvial processes within Valles Marineris occurs only locally. Small channels, chaotic material, and layered deposits were noted in early investigations of the canyons /7, 8/. The layered materials consist of two different rock types: a thick, competent, cliff—forming member, and a series of relatively thin, alternating, light and dark layered materials forming rounded hills and flat—topped mesas rising above the canyon floors (Figure 8). The layered floor deposits are believed to be waterlaid sediments that accumulated in large lakes within the canyons; their deposition was followed by episodes of catastrophic draining and flooding throughout the canyon systems /8/. Pmazonian System The Amazonian System includes the relatively young, featureless, volcanic materials within the northern lowland plains, and also sparsely cratered, fresh—appearing lava flows from the major volcanoes. The northern plains volcanic assemblage is composed of the Arcadia and Vastitas Borealia Formations, the major geologic units of the northern lowlands. They are separated from the more rugged, plateau—forming highlands by a highly dissected erosional scarp. The plains lava flows embay this highland boundary scarp in places and so do not represent an older residual surface exposed by erosion and scarp retreat /9/. The Arcadia Formation consists of six members, all lava flows, whose ages extend throughout Amazonian time. Lobate flow—fronts, pressure ridges, small volcanoes, and collapsed lava tubes or lava channels are co~mion in high—resolution pictures (Figure 2). Numerous small shield volcanoes and fault fissures are probably the sources of the lava flows of the Arcadia Formation.

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Fig. 8. Alternating dark and light bands may represent layered deposits (AHvl) that accumulated within large lakes that once filled chasmata of Valles Marineris. North at top. The Vastitas Borealis Formation consists of four members: mottled plains, knobby mottled plains, ridged plains, and grooved plains materials. The mottled plains unit occurs in a broad zone that nearly encircles the planet between about lats 500 and 700 N. Mariner pictures of this region were of poor quality because of atmospheric haze and high-sun angles. The mottled plains unit derived its name from its blurred appearance on the Mariner images combined with patchy high— and low—albedo contrasts. Viking pictures show that the contrasting light and dark patches are partly due to small impact craters with ejecta blankets brighter than intercrater areas. Another source of albedo contrast is produced by numerous conical hills with dark crestal areas; some hills have summit craters and may be of volcanic origin (Figure 9). Where the hills are very closely spaced, they are mapped as a separate, knobby member of the formation. The ridged and grooved plains members are characterized by raised and depressed sinuous to polygonal patterns, in some places resembling fingerprints. The ridge textures may reflect either primary or secondary forms; the grooves and troughs were probably produced by tectonisrn, contraction, or periglacial processes /10/ that have been enhanced by erosion. The Tharsis Montes Formation includes the giant shield volcanoes Arsia Mons, Pavonis Mons, and Ascraeus Mons (Figure 1) and their associated lava flows. The flows extend over a broad, gently arched plateau and have buried large parts of the highland terrain. The formation includes six members whose relative ages were determined by stratigraphic relations supported by crater—density determinations. The eruptive sequence of the members appears to have been nearly continuous from upper Hesperian to the end of Amazonian time. The Olympus Mons Formation also consists of six members, all of Amazonian age. The four oldest members form the large, relatively flat, semicircular aprons or aureoles surrounding the volcano. Their light color, wind—scoured surface, and flow-distribution patterns suggest that they may consist largely of pyroclastic material, possibly welded and nonwelded ignimbrites /11/. The two youngest members of the Olympus Mons Formation are lava flows that originated from the flanks and suniiiit of the volcano and from fissures in the adjacent plains. The plains member partly encircles Olympus Mans on the east and south, where it overlaps other members of the formation and much older flows of the plains (Figure 10).

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Three lava flows are included in the Alba Patera Formation. The oldest member (Hesperian) extends onto the lowland plains where it appears to be overlapped by the Arcadia Formation and in places by the Ceraunus Fossae Formation. The youngest member (Amazonian) partly buries the caldera and concentric faults around the sumit of the volcano. The Medusae Fomation extends along an east-west border zone between the highland plateau and lowland plains of Miazonis Planitia. It is interpreted /12/ to consist of ignimbrites, as it has smooth, flat to gently rolling surfaces that closely resemble welded and nonwelded ash—flow tuffs within the Basin and Range Province of the western United States (Figure 11). In places, where the softer, possibly nonwelded, upper parts

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Fig. 11. Composite photographs of nonwelded remnants of ignimbrites on Earth (E) and similar—appearing erosional remnants of postulated ignimbrites on Mars (M). Note: scales of pictures are vastly different. Martian mesas (M) about 20 km wide, those in the Basin and Range Province on Earth (E) about 1 km across. of the deposits have been stripped by wind erosion, the underlying material shows fracture patterns resembling complementary joint sets in terrestrial welded tuffs. Surficial materials of eolian, landslide, debris-flow, and periglacial origin occur throughout the western hemisphere of Mars. The largest mantling deposits are the smooth plains materials, which cover the floors of many craters, valleys, and other topographically low areas. The smooth plains deposits are more comon and are thicker in older craters than in younger ones. Crater counts indicate that their formation extended over a long time period. In some places they appear to consist of lava flows extruded from fractures in crater floors; in other areas they were famed by eolian and alluvial proc&ses. STRUCTURE Regional structural patterns on Mars are coanonly interrupted or overprinted by faults, fractures, and ridges that are oriented radial or concentric to specific features such as impact basins, volcanic centers, and upli fts such as the Tharsi s—Syria P1 anum rise. The study of stratigraphic relations of structurally defomed surfaces has provided new and more complete determinations of the tectonic history of the western equatorial region /13/. Tectonism in this region is mostly related to the Tharsis uplift, and is reflected by complex faulting of basement and fractured terrains of Noachian age (Figure 3a, b). Faulting continued on a reduced scale during Hesperian time, but is almost nonexistent in rocks of Amazonian age. Thus it appears that in the western equatorial region, tectonisrn culminated at a much earlier stage than volcanic activity /6/. The period of intense Tharsis faulting was followed by the extrusion of plains lava flows on Lunae Planum and on older rocks of the plateau sequence. The wrinkle—type ridges characteristic of the Lunae Planum and other lava flows (Figure 5) have trends that curve broadly around the Tharsis uplift. They mostly predate Valles Marineris and the Kasei Vallis river channel as well as the intense faulting around Syria Plarumi and in Tempe Terra. The origin of the ridges is controversial. In places, they are alined with and fill linear depressions interpreted to be grabens, suggesting an association with extensional tectonics. On the other hand, the ridges may have been formed as compressional folds associated with the Tharsis uplift /14, 15/, or by contraction of the planet’s surface /16/.

Global Geologic Mapping of Mars

The lava—covered plains units of the northern lowland region were formed after extrusion of the lava flows of the ridged plains, as they overlie these flows in the Chryse Basin. Postulated explanations for the origin of the regional topographic depression forming the lowland plains and their boundary scarp with the highlands include: crustal breakup due to a volume—expanding phase change in the mantle /17/. subcrustal erosion and crustal foundering caused by mantle convection /14/, and impact by a very large meteoroid /18/. Relatively recent structural deformation in the western region of Mars is shown by highangle normal faulting and expansion within Valles Marineris that has offset canyon wall and floor materials /7/. Northeast—southwest faulting along the crestal areas of Tharsis Montes has been the source of the youngest lava flows extruded from these volcanoes. The basal scarp around Olympus Mons is morphologically relatively young, but its origin is unknown. Some of the theories advanced for its formation include vertical tectonism /17/ and surface processes such as landsliding /19/, large—scale gravity slides /20/, and low— angle faulting /21/. CONCLUSION The geologic mapping of Mars has progressed from early prototype maps of local areas using images obtained from the Mariner 6 and 7 flybys, to the systematic mapping programs of Mariner 9 and Viking Orbiters. The Viking pictures, especially, have provided the improved resolution and quality necessary to recognize stratigraphic and structural relations in sufficient detail to justify remapping the planet at 1:15 million-scale. The rock units have been assembled in a vertical array that much better reflects their relative ages and position in sequence than was previously shown on the 1:25 million—scale map of Mars. No attempt has been made to estimate absolute ages of the geologic units. Present interpretations of the Martian impact crater flux with time show discrepanices of more than 1O~years for the same surface or event /22, 23, 24/. However, a wide range in morphological contrast occurs between lava flows assigned different ages based on their crater populations. This allows some discretion in judgement for selecting a crater timefrequency distribution curve or curves that seem to fit best the state of degradation of the flow units. Preliminary analysis using this method suggests that the Hesperian Era began about one billion years ago and extended to about 500 million years B.P. REFERENCES 1.

0. H. Scott and M. H. Carr, Geologic map of Mars, U.S. Geological Survey Misc. Geol. mv. Map 1—1083 (1978).

2.

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3.

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4.

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5.

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6.

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7.

K. R. Blasius, J. A. Cutts, J. E. Guest, and H. Masursky, Geology of the Valles Marineris: First analysis of imaging from the Viking 1 orbiter primary mission, J. Geophys. Res. 82, 4067 (1977).

8.

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9.

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D. H. Scott and K. L. Tanaka, Ignimbrites of Arnazonis Planitia region of Mars, J. Geophys. Res. 87, 1179 (1982).

13.

0. H. Scott and K. L. Tanaka, Mars: Paleostratigraphic restoration of buried surfaces in Tharsis Montes, Icarus, 45, 304 (1981).

14.

D. U. Wise, N. P. Golombek, and G. E. McGill, Tharsis province of Mars: Geologic sequence, geometry, and a defomation mechanism, Icarus, 38, 456 (1979).

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T. R. Watters and T. A. Maxwell, Ridge—fault intersections and Tharsis tectonics, in: Papers Presented to the Third International Colloquium on Mars, 270, Lunar and Planetary Inst., Texas, USA (1981).

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A. W. Gifford, Ridge systems on Mars, in: Planetary Geology, 219 (1981).

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T. A. Mutch, R. E. Arvidson, 3. W. Head, III, K. L. Jones, and R. S. Saunders, The geology of Mars, Princeton Univ. Press, Princeton, New Jersey, 1976.

18.

0. E. Wilhelms and S. W. Squyres, The Borealis basin of Mars, in: NASA Technical Memo. 86246, Reports of Planetary Geology Program—1983 (1984).

19.

R. N. C. Lopes, J. E. Guest, and C. J. Wilson, Origin of the Olympus Mans aureole and perimeter scarp, The Moon and the Planets, 22, 221, (1980).

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K. L. Tanaka, Mechanisms for the formation of Olympus Mans aureole deposits and basal scarp, Icarus, in press (1984).

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E. C. Morris, Structure of Olympus Mans and its basal scarp, in: Papers Presented to the Third International Colloquium on ~ 161, Lunar and Planetary Institute, Texas, USA (1981).

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W. K. Hartmann, Martian surface and crust: (1973).

23.

L. A. Soderblom, C. 0. Condit, R. A. West, B. M. Herman, and 1. J. Kreidler, Martian planetwide crater distributions: Implications for geologic history and surface processes. Icarus, 22, 239, (1974).

24.

G. Neukign and 0. U. Wise, Mars: A standard crater curve and possible new time scale, Science, 194, 1381 (1976).

NASA Technical Memo. 84412, Advances in

Review and synthesis, Icarus, 19, 550