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Preliminary Geological Assessment of the Northern Edge of Ultimi Lobe, Mars South Polar Layered Deposits

Murray, Bruce and Koutnik, Michelle and Byrne, Shane and Soderblom, Laurence and Herkenhoff, Kenneth and Tanaka, Kenneth L. (2001) Preliminary Geological Assessment of the Northern Edge of Ultimi Lobe, Mars South Polar Layered Deposits. Icarus, 154 (1). pp. 80-97. ISSN 0019-1035. doi:10.1006/icar.2001.6657.

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We have examined the local base of the south polar layered deposits (SPLD) exposed in the bounding scarp near 72°–74°S, 215°–230°W where there is a clear unconformable contact with older units. Sections of layering up to a kilometer thick were examined along the bounding scarp, permitting an estimate of the thinnest individual layers yet reported in the SPLD. Rhythmic layering is also present locally, suggesting a similarly rhythmic variation in environmental conditions and a recorded climate signal at least in some SPLD strata. Locally, angular unconformities may be present, as has been reported for the north polar layered deposits (NPLD) and may likewise imply intervals of subaerial erosion in the SPLD. The outcropping layers display a broad range of weathering styles and may reflect more diverse conditions of depositions, erosion, and diagenesis than might have been expected from simple aeolian depostion modulated only by astronomically driven climatic fluctuations. An unexpected finding of our study is the presence of locally abundant small pits close to the bounding scarp. These quasi-circular, negative, rimless features probably originated as impact craters and were modified to varying degrees by local endogenic processes, as well as locally variable blanketing. A nominal exposure age for the most heavily cratered region in our study area is about 2 million years, and the crater statistics appear consistent with those for the overall SPLD, although there are large uncertainties in the absolute ages implied by the crater size–frequency statistics, as in all martian crater ages. Another new finding is the presence of mass wasting features along the steepest portion of the retreating bounding scarp as well as a number of examples of brittle fracture, consistent with large-scale slumping along the bounding scarp and probably also ancient basal sliding. Both subhorizontal and high angle faults appear to be exposed in the bounding scarp, but the dips of the faults are poorly constrained. These fractures, along with the relatively undeformed layers between them, suggest to us that whatever horizontal motion may have taken place outward from the central cap region was accomplished by ancient basal sliding rather than large-scale glacial-like flow or ice migration by differential ablation, as proposed recently for the northern permanent cap and underlying NPLD. We have also obtained the first direct estimate of the regional dip of the SPLD, around 2–3° outward (northward) in one area.

Item Type:Article
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URLURL TypeDescription DOIArticle
Soderblom, Laurence0000-0002-0917-853X
Herkenhoff, Kenneth0000-0002-3153-6663
Additional Information:© 2001 Elsevier Science. Received November 30, 2000; revised April 20, 2001. We gratefully acknowledge the assistance of the following individuals: Tammy Becker, USGS, for help with ISIS; Trent Hare, USGS, for assistance in development and use of a Mars Polar Stereographic GIS; Anton Ivanov and Scott Anderson of JPL for assistance in the collocation of MOLA traces and MOC NA images. Our access to and use of MOLA data was facilitated by the current, user-friendly MOLA team website. We thank the following for preliminary manuscript review: Arden Albee, Caltech; R. Ashwin Vasavada, Caltech; Anton Ivanov, JPL; Asmin Pathare, UCLA; and especially Jeffrey Plaut and James Cutts of JPL for their detailed and very helpful formal reviews.
Subject Keywords:Mars, surface; geological processes
Issue or Number:1
Record Number:CaltechAUTHORS:20131211-152152382
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Official Citation:Bruce Murray, Michelle Koutnik, Shane Byrne, Laurence Soderblom, Kenneth Herkenhoff, Kenneth L Tanaka, Preliminary Geological Assessment of the Northern Edge of Ultimi Lobe, Mars South Polar Layered Deposits, Icarus, Volume 154, Issue 1, November 2001, Pages 80-97, ISSN 0019-1035, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:42966
Deposited By: Tony Diaz
Deposited On:17 Dec 2013 21:11
Last Modified:10 Nov 2021 16:31

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