Regional stratigraphy of the south polar layered deposits (Promethei Lingula, Mars): “Discontinuity-bounded” units in images and radargrams

Abstract

The Mars South Polar Layered Deposits (SPLD) are the result of depositional and erosional events, which are marked by different stratigraphic sequences and erosional surfaces. To unambiguously define the stratigraphic units at regional scale, we mapped the SPLD on the basis of observed discontinuities (i.e., unconformities, correlative discontinuities and conformities), as commonly done in terrestrial modern stratigraphy. This methodology is defined as “Discontinuity-Bounded Units” or allostratigraphy, and is complemented by geomorphological mapping. Our study focuses on Promethei Lingula (PL) and uses both high-resolution images (CTX, HiRISE) and radargrams (SHARAD) to combine surface and sub-surface observations and obtain a 3D geological reconstruction of the SPLD. One regional discontinuity (named AUR1) was defined within the studied stratigraphic succession and is exposed in several non-contiguous outcrops around PL as well as observed at depth within the ice sheet. This is the primary contact between two major depositional sequences, showing a different texture at CTX resolution. The lower sequence is characterized mainly by a “ridge and trough” morphology (Ridge and Trough Sequence; RTS) and the upper sequence shows mainly by a “stair-stepped” morphology (Stair-Stepped Sequence; SSS). On the basis of the observations, we defined two regional “discontinuity-bounded” units in PL, respectively coinciding with RTS and SSS sequences. Our stratigraphic reconstruction provides new hints on the major scale events that shaped this region. Oscillations in Martian axial obliquity could have controlled local climate conditions in the past, affecting the PL geological record

Automated reconstruction of subsurface interfaces in Promethei Lingula near the Martian south pole by using SHARAD data

“…Fig. 11(d) displays a depth map of the regional discontinuity (named here as AUR1) which is converted from the AUR1 DTM produced by Guallini et al (2018). According their study, the AUR1 should correspond to the SS2 shown in Fig.…”

“…Three sequences of layered deposits have been identified in this region. Between the top and the second layer sequences, an angular unconformity has been revealed by analysing high-resolution surface images (Kolb and Tanaka, 2006) and from analysing manually SHARAD radargrams Milkovich et al, 2009;Guallini et al, 2018). Although layer exposures can be found extending the South Polar Layered Deposits (SPLD) in the surface images, such as from Mars Orbital Camera (MOC) and Thermal Emission Imaging System (THEMIS) images, the extent of these layer sequences is observed to be interrupted in the SHARAD radargrams.…”

“…In SHARAD radargrams, the subsurface layer sequences are only revealed to be continuous within the Promethei Lingula region. Guallini et al postulated that the oscillations in Martian axial obliquity could have controlled local climate conditions in the past, leading to the current geological records according to their analyses of the geologic units in the region (Guallini et al, 2018).…”

“…(For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.) SHARAD radargrams by Guallini et al (2018). In this study, 146 SHARAD radargrams are used.…”

“…Compared to the AUR1, the SS2 depth map shows more detailed variation which may due to the difference in interpolation methods between Kriging and TIN interpolation. This difference can be also caused by less radargrams used for interpolating the AUR1 although more than 600 SHARAD radargrams are inspected in the Promethei Lingula region, which is larger than the AUR1 extent according to Guallini et al (2018). Besides, the AUR1 DTM is derived not only by subtracting depth from MOLA DTM, but also by further subtracting the average elevation in each SHARAD orbit.…”