DO BOULDER DISTRIBUTIONS ON LOBATE DEBRIS APRONS INDICATE REGIONAL-TO-GLOBAL SYNCHRONICITY IN GLACIAL FLOW RATES?: SEDIMENT COVER PATTERNS RESULTING FROM MARTIAN GLACIATION. J. S. Levy1, C. I. Fassett2, M. Tebolt1 1Colgate University, 13 Oak Ave., Hamilton, NY, jlevy@colgate.edu, 2NASA Marshall Space Flight Center, Huntsville, AL.

Introduction: Landforms including lineated valley fill (LVF), lobate debris aprons (LDA), and concentric crater fill (CCF) are the dominant debris-covered glacial landforms on Mars, covering ~7 x 105 km2 of the mar-tian surface between ±~30-50º latitude [1], representing a global water-equivalent layer 0.9-2.6 m thick [1,2].

These landforms represent an important component of the Amazonian [3] water ice budget, however, be-cause small craters (diameter D≤ 0.5-1 km) are poorly retained on the surface of CCF, LDA, and LVF, and, since the glacial landforms are geologically young, it is challenging to reliably constrain individual ages in or-der to determine how quickly the glaciers accumulated [15]. Modeling investigations of ice flow under martian conditions suggest that LDA could accumulate and flow to their full extent in as little as ~500 kyr [4] or could require over 100 Myr [5]. A fundamental question for individual glaciers is whether ice deposition and flow occurred episodically during a few, short instances, or whether glacial flow was quasi-continuous over a long period. Because glaciation is thought to be controlled largely by obliquity excursions [6,7], a larger question is whether glacial deposits on Mars exhibit regional to global characteristics that can be used to infer synchro-nicity of flow or degradation.

Because rocks exposed on Mars’ surface are known to breakdown and weather to a finer grained regolith, we hypothesize that observable boulder size may de-crease as a function of position down-glacier, with age increasing with distance between the accumulation zone

of these glaciers to their termini. Debris movement and

boulder breakdown thus could act as a “ticker tape” ad-vancing out of the accumulation zone such that boulder population on the debris surface is sensitive to the epi-sodicity and intensity of glaciation as a function of time. Alternatively, if the boulder breakdown timescale is long compared to glacial flow rates, or if boulders are mostly entrained englacially, no such signal may be ob-served.

Methods: We mapped boulder size-frequency dis-tribution over 16 LDA and CCF landforms. Boulders were mapped manually on 25 cm/px HiRISE images along a flow-line determined through observations of CTX and HiRISE stereo DEMs generated for each site using ASP [8-10]. Boulder measurement sites are widely distributed over the martian surface, and include examples in Protonilus/Deuteronilus, eastern Hellas, and Mareotis Fossae.

Results & Discussion: Boulder size is more strongly correlated with density than with distance down glacier (Fig. 1). Boulder size, both median and 95th percentile (in 50 m distance bins) are consistently variable, with large clasts appearing not only at the up-per end of the glacial landform.

Intriguingly, the most notable signal observed be-tween sites is the persistence of ~1-5 concentrations or “bands” of boulders, which are apparent in plots of boulder distribution with distance down-glacier (Fig. 2). The presence of zones of dense boulder cover banding in lobate debris aprons separated by thousands of km suggests the possibility that these LDA are recording a

regional or larger-scale climate signal associated with

Figure 1. Boulder count, surface slope from stereo DEM, and boulder size for site J, ESP_016132_2300. Boulder size does not decrease with distance from the glacier headwall. Rather, steep areas are associated with increased boulder density and size.

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ice deposition and flow and/or geological weathering rates. Such bands of dense clast cover could emerge from periods of slow ice flow (little accumulation), or periods of rapid erosion.

One intriguing implication of the absence of a mon-otonic boulder size reduction with distance down-glac-ier is that is suggests that englacial flow of debris [11] may be responsible for transporting large clasts through the glacier with minimal weathering or comminution. Although internal radar reflectors are not directly ob-served in LDA [12], the emergence of large, uncommi-nuted clasts near the base of the glaciers is consistent with a sublimation lag origin for at least part of the de-bris layers, rather than continuous conveyor-style movement of rocky lag downslope from headwall mass

wasting. If concentrated boulder bands are associated with englacial debris bands intersecting the surface of the glacier, they may reflect accumulation of debris dur-ing periods of reduced ice accumulation [13,14], provid-ing a regional link between glaciation and obliquity.

Acknowledgements: This work was supported in part by NASA award 80NSSC18K0206.

References: [1] Levy, J.S. et al. (2014) JGR, 119, 1936–1949. [2] Karlsson, N.B. et al. (2015) GRL, 42, 2627–2633. [3] Fassett, C.I. et al. (2014) Geology, doi:10.1130/G35798.1 [4] Fastook, J.L. & Head, J.W. (2014) P&SS, 91, 60–76. [5] Parsons, R.A. et al. (2011) Icarus, 214, 246–257. [6] Forget, F., et al. (2006) Science, 311, 368–37. [7] Head, J. W. et al. (2006) EPSL, 241, 663–671. [8] Moratto, Z.M., et al.

(2010) 41st LPSC, #2364. [9] Beyer, R. A., et al. (2014) 45th LPSC, #2902. [10] Broxton, M. J. & Ed-wards, L. (2008) 39th LPSC, #2419. [11] Mac-kay, S. L., et al. (2014) JGR, 119, 2505–2540. [12] Holt, J. W. et al. (2008) Science, 322, 1235–1238. [13] Holt, J. W., et al. (2016) EGU 18, EGU2016–11252. [14] Mackay, S. L. & Marchant, D.R. (2017) Nat. Comm., 8, 1–12. [15] Baker, D.H. & Carter, L.M. (2019) Icarus, 319, 745-769.

Fig. 2. Down-glacier profiles of boulder location (x-axis) and distance from the profile centerline (y-axis) for 15 LDA and/or CCF sites across martian mid-latitudes. Boul-der “bands” are present at all locations, and commonly are present at or near similar length-scaled locations along the profile (i.e., LDA are of variable length, but have been placed on a common scale to permit site-to-site comparison). Gray eye-guides have been added to each column of plots, and should not be interpreted to reflect precise band loca-tions. Rather, they highlight that even arbitrarily grouped boulder profiles share nota-ble similarities in the spacing of boulder concentrations.

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