Mars North Polar Layered Deposits Stratiagraphic Units: Top to Bottom
||Figure 1. Illustration of identifying layer sequences in MOC images. Colored lines denote reference layers which comprise the ULS and LLS.1
Dr. Kathryn Fishbaugh
Using Mars Orbiter Camera (MOC) and Mars Orbiter Laser Altimeter (MOLA) data, we have been studying the Mars north polar layered deposits (PLD) and underlying basal unit (BU), which make up the broad Planum Boreum dome. The upper PLD contains at least two correlatable visible layers sequences: the upper layer sequences (ULS) and the lower layer sequences (LLS)1. We found their mass balance patterns (Balance between accumulation and ablation at any given point on a concurrent surface) to be different, indicating the presence of an unconformity. There are no noticeable trends for the ULS accumulation rate1. In contrast, the LLS accumulation rate decreases with distances from the pole, as exhibited by terrestrial ice sheets1. The accumulation rare of the NPLD has not been constant over time1. We also found that mass balance patterns, not flow, have shaped the structure of the ULS and LLS layer sequences.
The BU, which is separated from the PLD by a major, erosional unconformity, represents a more complexly layered unit than the overlying PLD, containing evidence of sandy, cross-bedded layers.
||Figure 2. Example of basal unit outcrop in Olympia Planitia. (A) Geologic interpretation of image. The black line indicates location of the MOLA profile used for the geologic cross-section. (B) Approximate geologic cross-section corresponding to the geologic interpretation in (A) using a MOLA profile. (C) Portion of MOC image being referred to in (A).2
In comparison to the polar layered deposits, much less is known about the underlying basal unit. Restricted to studying the two outcrops in Olympia Planitia and Chasma Boreale2, we aimed to identify the basal unit’s major characteristics and origin. BU exposures at the two outcrops reveal a dark layers remnant of differential erosion2, primarily by wind. The BU is the primary, if not sole source of the northern polar sand dunes and ergs2. Other major BU characteristics we have determined include it has a broad, mound shape with a thickness of ~ 600 m, a volume of 3x105 km3, makes up 7% of the greater Vastitas Borealis Formation region, has little to no deformation, and at least in Olympia Planitia, has low albedo layers of varying thickness2.
Four possible origins have been previously suggested for the BU: 1) outflow channel/oceanic deposits, 2) basal ice. 3) paleopolar deposits, or 4) eolian deposits2. Our studies support the eolian deposit theory initially concluded by Brybne and Murray in 20022. Meaning, during the Early to Late Amazonian, migrating sand was mixed with water ice, forming a dark, sandy deposit2. Concurrently, there were no traditional polar layered deposits though perhaps small caps fluctuating in size contributed material2.
1. Fishbaugh and Hvidberg. (2006), Martian north polar layered deposits stratigraphy: Implications for accumulation rates and flow.
2. Fishbaugh and Head. (2005), Origin and characteristics of the Mars north polar basal unit and implications for polar geologic history.
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