Constraints on the geologic history of "Home Plate" materials provided by clast morphology and texture

Abstract

We quantified and classified the size, shape, roundness, and texture of 333 loose surface particles (clasts) along the Spirit rover traverse from Martian solar days 750 to 1889. These data provided clues to origin, transport mechanisms, association with geologic units, and other alteration processes. Clasts average 15 mm in size and 0.73 sphericity, and display a low degree of roundness; textures include vesicular, smooth and rough flat-faceted, and granular, platy clasts. Characteristics are comparable to those for clasts outside the "Home Plate" region. Mapped geologic units around "Home Plate" are characterized by distinctive clast morphologic parameters and only minor adjustments were made to unit boundaries and definitions mapped by others; no new units are suggested by the data. Consistent roundness values seen from unit to unit, coupled with the correlation between unit boundaries and clast characteristics in and outside "Home Plate," indicate that no appreciable transport has occurred subsequent to fragmentation from outcrop and emplacement. Rather, clasts have been emplaced through volcanic activity and/or impact and have been subjected to weathering processes since that time. We interpret vesicular and flat-faceted clasts (texture types 1-3) to be derived from Mitcheltree ridge basalts and the presence of these clasts to provide a general outline of the original extent of this unit. Platy, granular-textured clasts (texture type 5) are derived from the upper Home Plate unit. Copyright 2010 by the American Geophysical Union.