Lunar and Planetary Science XLVIII (2017) 1644.pdf

GEOLOGIC MAP OF WEST CANDOR , : UNCONFORMITIES. B. K. Lucchitta, U. S. Geological Survey, Astrogeology Science Center, Flagstaff, AZ 86001. [email protected].

Introduction: Interior layered deposits (ILD) in west it has an unconformity at its base but, in the lowlands to occur in the dome Ceti Mensa and in the the , lacks the expected unconformity with the over- surrounding lowlands. The geology of the area is summa- lying cap-rock group (see below) near its top. rized in the index map (next page), which is a simplified Upper unconformity: This major unconformity is version of the map in Lucchitta [1]. The ILD are com- well expressed and has been recognized by many workers posed of three major groups, from the bottom up (1) a [5-8]. Kite [7] even measured its undulating surface. The core group (co, brown tones), (2) a rim-rock group (la, unconformity cuts deeply across the core and much of the blue tones) and (3) a cap-rock group (ca, tones). rim-rock groups after both were deformed and after the Landslides (ls, yellow) line the north wall of the trough. reentrants were eroded. The stripped surface was then A dark mantle (da, purple) occurs in the SE corner of the covered by the cap-rock group, consisting of a ledgy unit map. Major unconformities separating the groups are the at the base, a fluted (yardang) unit above it, and a scal- subject of this report. loped unit near the top [1]. These units are separated Lower unconformity: the core group is generally from each other by smaller unconformities. The cap-rock light toned and has pink to light ochre hues in HRSC group occurs mostly on the southern flank of Ceti Mensa, color data. Its base is not exposed. The unit looks mas- but the fluted unit extends into the lowlands surrounding sive, but in high resolution shows regularly bedded, fine, the mensa. There the unconformity at its base locally is fairly uniform, faceted layers. In the SE reentrant the ex- less obvious. The fluted unit also covers wall-rock land- posed core layers and rim-rock layers are conformable slides. Figure 3 shows where the ledges unit overlies a and their attitudes agree with the mensa slopes, suggest- stripped surface of the resistant, bumpy unit. Figure 4 ing an origin by draping for both units. However, a dark- shows where the scalloped unit overlies contorted beds er, more distinctly layered upper core unit is exposed in (yellow lines) of the lower-layered unit. the NW reentrant. Figure 1 shows that tilted beds (yellow Late unconformity: Dark mantles occur throughout lines) of this unit are truncated (blue line) by lower- west Candor Chasma and lie unconformably on all other layered beds of the rim-rock group. This unconformity units. Many have a thin sequence of light layered beds suggests that perhaps an undetected disconformity exists underneath. Dark mantles are preserved only as patches in the SE reentrant as well. Overall, there appears to be in most places. Their dark tops most likely are locally evidence for an erosional interval between the core and derived windblown dust, but the underlying layers may rim-rock units. belong to a late phase of ILD emplacement. Figure 2 Middle unconformity: Rim-rock group layers (la on shows the dome Nia , which has an extensive the map) form major cliffs along the upper rim of the two dark-mantle cap (da on map), which unconformably (red reentrants. They underlie much of the top of Ceti Mensa line) truncates tilted beds (yellow lines) of both core and and conform to the slopes of the dome. The lower-layers lower-layered beds. unit is mostly evenly bedded and its outcrops dominate Conclusion: All units within Ceti Mensa and the sur- the eastern and northern slopes of the mensa. The unit is rounding lowlands are composed of layers, but their char- overlain by a discontinuous resistant unit with a rugged, acteristics are sufficiently distinct to suspect variations in bumpy surface. This unit pinches out on top of Ceti Men- their depositional environment. All layers, including the sa and elsewhere, most likely due to erosion. The overly- exposed layers of the core group, conformed to past or ing knobby unit commonly shows contorted layers littered current slopes on the mensa, suggesting strongly that with knobs [2]. It extends from near the top of Ceti Men- some form of airfall process formed the mounds. The sa into the lowlands to the W, SW, and NW, where the erosion during the intervals represented by the uncon- layers lap up onto the trough walls [3,4]. Figure 1 shows formities is substantial, indicating that the ILD formed that this unit has a major unconformity (red line) at its over an extended period of time. base. The knobby beds rest on a platform covered by dark References: [1] Lucchitta, B. K. (2015) LPS XLVI, Abstract wall-rock debris (outlined in green), which is cut across #1550. [2] Chan M. A. et al. (2010) Icarus, 205, 138–153. [3] beds (yellow lines) of the lower-layered unit. The ero- Okubo C. H. et al. (2008) JGR, 113, E12002, sional episode was long enough to allow for the planing doi:10.1029/2008JE003181. [4] Okubo C. H. (2014) U. S. Geol. of the lower-layered rocks and the covering of the plat- Survey, Sci. Inv. Map 3309. [5] Mangold N. et al. (2008) Icarus, form with dark debris, before renewed sedimentation took 194, 519–543. [6] Fueten F. et al. (2008) JGR, 113, E10008, place. The knobby unit may actually be a transitional unit doi:10.1029/2007JE003053. [7] Kite E. S. et al. (2016) JGR, 121, between the rim-rock and overlying cap-rock groups. Its doi: 10;1002/2016JE005135. [8] Okubo C. H. and Gaither, T. A., stratigraphic affiliation is somewhat ambiguous, because (in press) U.S.Geol. Survey, Sci. Inv. Map 3359. Lunar and Planetary Science XLVIII (2017) 1644.pdf

Fig. 1. Expt. Google Mars CTX obl., 1.5 x vert. exag., 20 km wide. Fig. 2. Expt. Google Mars CTX obl., 1.5x, 16 km wide. Fig. 3. CTX P04_002696, 6 km wide. Fig. 4. Expt. Google Mars CTX obl., 3x, 4 km wide. Index map, condensed from [1]. Pink numbers: fig. locations. Ls landslides, co core group, la rim-rock group, ca cap-rock group, da dark mantle. SE and NW denote reentrants. Orange line surrounds Ceti Mensa. .