Lunar and Planetary Science XXXVIII (2007) 1856.pdf

STRATIGRAPHY AND LANDSCAPE EVOLUTION OF CHASMA BOREALE, MARS. K. L. Tanaka1 and M. C. Bourke2, 1Astrogeology Team, U.S. Geological Survey, Flagstaff, AZ 86001, [email protected], 2Planetary Science Institute, Tucson, AZ.

Introduction: Chasma Boreale forms a broad, ar- 3 units (ABb1-3), also known as polar layered deposits cuate valley cut into Planum Boreum, the north polar (units 1 and 2) and residual ice (unit 3), bury all the plateau of Mars. Proposed origins for this feature in- aforementioned units. However, unit ABoc apparently clude: (1) erosion by subglacial floods (jökulhlaups), was emplaced only locally, and its relatively bright perhaps induced by pressure melting or magmatism layers may be comparable to bright basal layers of unit [e.g., 1-2], and (2) eolian erosion, including topog- ABb1 exposed elsewhere. HiRISE images (e.g., raphic control [e.g., 3-4]. Each of these interpretations PSP_001334_2645) show that units ABb1-2 and ABoc reflects on whether or not Planum Boreum has experi- are intensely fractured in places. enced glacial flow. New high-resolution image data Bright, finely layered deposits covering the floor of acquired for parts of Chasma Boreale, including Mars Tenuis Cavus are partly covered by dunes and appear Reconnaissance Orbiter HiRISE and CTX images as to embay the Olympia Cavi unit and to underlie unit well as ongoing geologic mapping results, further elu- ABb2 (see CTX-8B). We have tentatively named these cidate the eolian history of this feature. deposits the Chasma Boreale unit (ABcb). At present, Topography: Planum Boreum forms a subcircular the age of this unit relative to ABb1 is uncertain and it plateau ~1000 km across and upwards of 2500 m thick either formed during or after deposition of unit ABb1. that is dissected by many spiral troughs 10s of km This unit possibly includes layered deposits tens to a wide and 100s of km long. Chasma Boreale itself few hundred meters thick that extend from western forms a huge trough oriented tangential to the north Boreum Cavus southwestward across much of Chasma pole. However, its floor is made up of a low plateau, Boreale’s floor. Hyperborea Lingula ~200 to 350 m above the adjacent Surficial materials: Dark dune fields occur in the plain of Vastitas Borealis. Two partly enclosed arcu- cavi and as an elongated sand sea (Hyperboreae Un- ate depressions, Boreum and Tenuis Cavi (eastern- dae) within Chasma Boreale. The dunes appear to most), mark the head area of Chasma Boreale. source from the dark unconsolidated parts of the Stratigraphy: Recent mapping results [4-5] along Olympia Cavi unit exposed in the steep, eastern scarps with investigation of newer data sets (reported here) of Boreum and Tenuis Cavi. The surface of the dunes reflect greater complexity in the stratigraphy of displays transverse ripples in HiRISE images; the pro- Planum Boreum than previously recognized. The sur- files of these small ripples appear either symmetric or rounding plain is covered by the Early Amazonian steeper on the sides facing the primary down-wind Vastitas Borealis interior unit (ABvi), which may be direction. Orientations are consistent with local dune- earlier outflow discharges that were subsequently mas- induced wind deflection. The dunes are bordered by sively reworked, perhaps by periglacial-like processes sand sheet deposits that are rippled and have small [6]. Thickly-layered materials that make up Hyperbo- barchans in the leeward cusps of the large barchans. rea Lingula, Rupes Tenuis, and the lower parts of the The orientation of the smaller dunes indicates reversed northern walls of Chasma Boreale are mapped as the wind transport in this location. In some places, these Early Amazonian Rupes Tenuis unit (ABt), which may sand sheet deposits infill polygonal cracks. Large lin- be the result of locally reworked unit ABvi. materials. ear dunes have both rippled flanks and apparent active Unit ABt retains a significant density of superposed grain flow on flanks. These dune and ripple arrange- impact craters, many of which have what appear to be ments suggest multi-directional winds. pedestal ejecta forms indicative of ejecta armoring a The dunes are mostly underlain by a rippled layer mantle. The mantle may be remnants of ephemeral that may be a sand sheet; the ripples are inferred to be deposits that perhaps were emplaced and removed aligned perpendicular to the primary wind direction repeatedly throughout much of the Amazonian [4]. (down-chasma). Locally, MOC images show hollows Following what may have been a considerable hia- in the sheet that appear to be former locations of dunes tus, dark unconsolidated material interbedded with [7]. The ripples appear to be indurated. Similar ripples bright, fractured layers were deposited, perhaps only occur on the eastern scarp of Boreum Cavus; these are locally. We call this the Olympia Cavi unit (ABoc), locally buried by blocky slide deposits shed from unit which may be Late Amazonian given its paucity of ABb1. These indurated ripples have a degraded appear- craters. HiRISE images show cross-bedding in the ance in places, and do not show signs of reorientation layers, which are darker in color near the base of the in response to slide and dune features that override unit. Finely layered materials of the Planum Boreum 1- them. However, some dark, smooth ripples that occur Lunar and Planetary Science XXXVIII (2007) 1856.pdf

on the cavi scarps have crests orientated in the down- mored pedestals of impact craters, which suggests that slope direction and appear related to cross-slope unit ABt was easily eroded in earlier times. Also, some winds. They interact locally with boulders and thus layers of unit ABt appear relatively thicker and more may be active. resistant. This indicates that such a layer may cap Hy- Brighter toned dunes, broad ridges, and apparent perborea Lingula, which could account for the preser- mantle material overlie part of the margins of Planum vation of this plateau. Boreum above the head scarp of Boreum Cavus. These The crater pedestals appear to be important as to- may be unconsolidated dust and sand deposits eroded pographic buttresses in the later accumulation and/or from the Planum Boreum units. preservation of unit ABb1 on the northwestern flank of Dark veneers on Planum Boreum may be made up Chasma Boreale. The southern flank of Chasma Bo- of dust eroded from older deposits, including a local reale follows the exposed irregular but arcuate eastern dark layer at the base of unit ABb2 [8]. Dark material margin of Hyperborea Lingula. The Hyperboreae Un- also appears as a possible layer or lens in a pit along a dae dune field extends from Boreum Cavus across the graben wall within the residual ice (HiRISE lingula, stepping across the exposed scarp connecting PSP_001513_2650). with and perhaps feeding extensive dune fields in the Erosional/degradational features: Surfaces of plain south of Chasma Boreale. what may be the Rupes Tenuis unit (ABt) on the floor Unit ABoc appears to have been emplaced only lo- of Tenuis Cavus display subdued impact crater forms, cally around Planum Boreum, including in the eastern, including ejecta blankets. The surfaces are finely southern, and northern rims of Boreum and Tenuis cracked (as seen in HiRISE) and may be highly modi- Cavi. It may be that these were areas where sand-rich fied and altered. On the margin of Hyperborea Lin- material tended to collect between phases of bright gula, unit ABt displays large polygonal troughs that layer accumulation produced by precipitation of ice include zones relatively resistant to erosion. and dust. The pre-existing northern scarp of Chasma HiRISE and MOC images reveal fields of aligned Boreale likely served to channel katabatic winds across troughs and grooves in surfaces of layers and uncon- the chasma floor area, perhaps preventing significant solidated dark deposits in the west-facing head scarps accumulation of units ABb1-2. Minor infilling of and plateaus above them of Boreum and Tenuis Cavi. Chasma Boreale either coeval with unit ABb1 or be- They generally have smooth margins in the dark de- tween units ABb1-2 produced the unit ABcb. This unit posits and rugged margins in consolidated layers. They generally appears to be evenly layered, without cross- are oriented parallel to the primary, regional wind di- bedding and dark sandy layering as occurs in unit rection as indicated by dune forms; they also parallel ABoc. Thus unit ABcb was emplaced during an epi- broad troughs in Planum Boreum above the cavi head sode in which dark sandy material was virtually absent scarps. We interpret these features to be degraded yar- in the cavi, perhaps as a consequence of low wind en- dangs. They do not follow the polygonal cracking ori- ergy and/or coeval entrapment of dark sandy material entations and thus may be relict features formed during in other locations. accumulation of the unit. Conclusions: Our observations of exposed strati- Landscape evolution: Topography and stratigra- graphy and extant eolian landforms within Chasma phy appear to have exercised the greatest influence in Boreale indicate that the margins of an earlier plateau the formation of Chasma Boreale, controlling erosional made up of the eroded Rupes Tenuis unit likely con- and depositional processes. During the Early Amazo- trolled winds patterns that prevented significant infill- nian, emplacement of unit ABt formed a plateau that ing of the chasma, while its flanks became overlain by may have exceeded ~1300 m in thickness, perhaps finely layered deposits. This origin is similar to that extending south to ~65ºN. This unit may have been the proposed for Mars’ south polar chasmata [9]. result of reworking of Vastitas Borealis units by sedi- References: [1] Clifford, S. M. (1987) J. Geophys. mentary diapirism and volcanism in the Scandia re- Res., 92, 9135-9152. [2] Fishbaugh K. E. and Head J. gion, which may also be the origin of the Scandia unit W. III (2002) J. Geophys. Res., 107, that surrounds part of Planum Boreum [5-6]. 10.1029/2000JE001351. [3] Howard, A. (2000) As a result of erosion, the form of this early polar Icarus, 144, 267-288. [4] Tanaka K. L. et al. (2006) plateau became highly irregular, as represented largely LPS XXXVII, Abstract #2344. [5] Tanaka K. L (2005) by the present surface of unit ABt, much of which is Nature, 437, 991-994. [6] Tanaka K. L. et al. (2005) buried beneath younger, finely layered units. Thus USGS Map SIM-2888. [7] Mullins K. F. et al. (2006) much of the early polar plateau’s original outline and LPS XXXVII, Abstract #1998. [8] Rodriguez J.A.P. form are uncertain. Along the SW margin of Chasma and Tanaka K. L. (2007) this volume. [9] Kolb E. J. Boreale, the relict surface of unit ABt consists of ar- and Tanaka K. L. (2006) Mars, 2, 1-9.