46th Lunar and Planetary Science Conference (2015) 1210.pdf

HIGH-RESOLUTION STRUCTURAL AND GEOLOGIC MAPPING IN CANDOR . C. H. Okubo, U.S. Geological Survey, 1541 E. University Blvd., Tucson, AZ, 85721, [email protected].

Introduction: High-resolution geologic and struc- Additional evidence supporting extant shallow tural mapping of four key sites in west Candor Chasma groundwater during sedimentation is found in the form is nearing completion, and new mapping is commenc- of soft-sediment deformation. Contorted bedding is ing at three sites in east Candor Chasma (Fig. 1). These observed in most map areas. More spectacular are the high-resolution maps are constructed using digital ele- first rigorous identifications of injectite megapipes on vation models (DEMs) and orthorectified imagery con- , the Candor Colles. These injectites formed con- structed from stereo HiRISE observations. The maps temporaneously with several newly identified fault have been, or will be, formally published through the zones along the southern flank of Ceti . Their US Geological Survey’s Scientific Investigations Map contemporaneous nature indicates that seismicity due series at 1:18,000 scale. This paper presents a synthesis to slip along these faults triggered liquefaction of the of results from west Candor and introduces the three surrounding water-saturated, poorly lithified sediments new sites in east Candor. and formation of the injectite megapipes. West Candor Chasma: Mapping in west Candor Structural evidence of slope failure of the sedimen- Chasma focused on the separate regions of central tary deposits is found in each of the four west Candor Candor Colles, southeast and west Ceti Mensa and east Chasma map areas. These landslides are manifest as Candor Sulci. The central Candor Colles map has been circular failures, translational slides or soil spreads. published [1] and the remaining three maps are cur- The aforementioned fault zones along the southern rently under review. These maps focus on the stratified flank of Ceti Mensa are the basal surfaces of a land- sedimentary deposits within the chasma – the canoni- slide complex. Thus these slope failures are interpreted cal Late to Early “Interior de- to have triggered the formation of the Candor Colles posits” of [2] and others. and associated soft-sediment deformation. Collectively, mapping relationships reveal that The faults along southern flank of Ceti Mensa are landslides of chasma wall rock (Ceti Labes and Candor also notable for providing the first recognized expo- Labes) predate deposition of the local sedimentary sures of a fault damage zone on Mars. These damage deposits. This and other onlap relationships establish zones are 100–200 m wide and surround the faults that these sedimentary deposits accumulated within the along most of their exposed length. Structural analysis chasma, and rule out alternative interpretations that the of the damage zone architecture indicates that it is pre- chasma formed after the sediments were deposited. dominantly composed of deformation bands arranged Bedding morphologies and onlap relationships in a ladder structure [3] geometry. These observations within the sedimentary deposits indicate that they ac- indicate that the sediments were porous, granular, and cumulated in wet playa-like environments. Dune bed- poorly indurated at the time of landsliding. forms are present but rare, being only clearly observed The sedimentary deposits also exhibit evidence of the west Ceti Mensa map area. The majority of the recurrent periods where sedimentation ceased and eoli- strata have morphologies consistent with sand sheets. an erosion was widespread. These hiatuses in deposi-

Figure 1. Locations of the maps discussed in this paper. Background is a THEMIS daytime infrared mosaic. 46th Lunar and Planetary Science Conference (2015) 1210.pdf

tion are recorded as prominent unconformities that can derived from the chasma wall, HiRISE observations be traced through large areas of west Candor. The sed- show that the deposits are well stratified. Thus, refined imentary deposits are divided into geologic units based mapping at the HiRISE scale may lead to these depos- on these regional unconformities – the units are allo- its being reinterpreted as sediments. This map area also stratigraphic. Eleven sedimentary units are mapped in encompasses a part of the proposed chasma-related total amongst the four map areas and some units are normal fault that bounds the north side of east Candor observed in multiple maps. Many more allostratigraph- Chasma (c.f., [4]). As such, this map area offers an ic divisions are observed outside of the map areas, re- opportunity to document any evidence of chasma- cording more than twenty-five periods of regional ero- related faulting, and any superposition relationships sion. The cause of the transition from subaerial deposi- therein, within the wall rock and stratified deposits. tion to massive eolian downcutting has not yet been Map area 2 falls along the proposed chasma-related constrained through mapping. normal fault that bounds the side of east Candor These periods of regional eolian erosion are inter- Chasma, and encompasses Hesperian- and Amazonian- preted to have contributed to the observed slope fail- aged interior deposits of [2]. Given this wide range of ures. The regional unconformities dip at steeper angles unit ages, quantifying the styles, distribution, and su- than bedding does. Therefore eolian erosion and perposition relationships of deformation here will help downcutting resulted in steepening of the long- to constrain the timing and duration of any chasma- wavelength paleotopography, thereby contributing to a related faulting in east Candor. Some workers suggest destabilization of the sediment slopes. Seismicity from that the older layered deposits contain chasma-related near by impacts may have also triggered landslides. faults, which are buried by younger layered deposits in Landsliding occurred contemporaneously with sed- west Candor (e.g., [5,6]). Thus map area 2 provides an iment accumulation. The central Candor Colles and optimal location to test for such relationships in east southeast and west Ceti Mensa map areas show clear Candor. HiRISE observations here also reveal possible evidence of landslides of sedimentary deposits being morphologic evidence of inverted fluvial channels deflated along regional unconformities, and then bur- within some stratified deposits. Documenting any oc- ied by subsequent sedimentary units. currences of such landforms would provide new in- East Candor Chasma: Though insightful, the re- sight into their controlling depositional environments sults from the relatively small areas studied in west recorded in east Candor. Candor Chasma offer only a glimpse into the broader Map area 3 is located along the same putative geologic history of . In order to realize chasma-related normal fault as map area 2. This map the full potential that high-resolution structural and area encompasses a spur of the chasma wall rock, as geologic mapping have for unraveling the geologic well as a variety of stratified deposits. Of the three map history of Valles Marineris, additional high-resolution areas in east Candor, this area is most analogous to the mapping must be undertaken in other chasmata. west Candor map areas, as it contains Hesperian- and East Candor Chasma is selected as the next logical Amazonian-aged interior deposits and -aged focus area. This region has received relatively little wall rock. Observations here will help to evaluate the attention with respect to post-Viking era mapping de- broader applicability of the general timing and deposi- spite being physiographically comparable to west Can- tional environments found in west Candor Chasma. dor Chasma and containing morphologically similar Summary: High-resolution structural and geolog- stratified deposits. Many of the same geologic units are ic mapping in west Candor Chasma reveals that the identified by [3] in both the eastern and western parts stratified deposits on the floor of the chasma are basin- of Candor Chasma. Thus results from this expanded filling sediments that were deposited in a wet playa- mapping effort can be used to develop an integrated like setting. These sediments were sculpted by occa- geologic history of Candor Chasma as a whole. sional periods of regional erosion, landsliding, and Mapping in east Candor Chasma will focus on soft-sediment deformation. This research is now being three separate areas that have yet to be given formal extended into east Candor Chasma, with the goal of geographic names. Just as with the west Candor Chas- developing a more complete and detailed geologic ma mapping, these maps will concentrate on the strati- history for this part of Valles Marineris. fied deposits within the chasma and their relationship References: [1] Okubo C.H. (2014) USGS SIM to the chasma wall rock and its derived deposits. 3309. [2] Witbeck N.E. et al. (1991) USGS Map I- Map area 1 encompasses the contacts between an 2010. [3] Schultz R.A. and Balasko C.M. (2003) GRL, inselberg of chasma wall rock and stratified deposits 30, 2033. [4] Schultz R.A. and Lin J. (2001) JGR, 106, on the floor of east Candor Chasma. Though [2] inter- 16549–16566. [5] Fueten F. et al. (2006) GRL, 33, pret most of the deposits here as landslide material L07202. [6] Fueten F. et al. (2008) JGR, 113, E10008.