HAND-LENS SCALE SEDIMENTARY AND TEXTURAL FEATURES IN MUDSTONES AND THEIR ENVIRONMENTAL IMPLICATIONS

Due to the fine-grained nature of mudstones, the hand lens has long been an indispensable tool for mudstone researchers. In a survey of shales from a variety of sedimentary settings and geologic ages we can document an assortment of millimeter to centimeter scale sedimentary features that can be related to depositional processes. For example, internal laminae show a large range in thickness and lamination styles (even, discontinuous, lenticular, wrinkled etc.), which can represent quiet settling, sculpting of the sediment surface by bottom currents, and growth of microbial mats respectively. Internal lamina features, e.g. grading (a), random clay orientation (b), preferred clay orientation (c), sharp basal contacts (d), and sharp top contacts (e) may be interpreted as indicative of (a) event-sedimentation (e.g. floods, storms, turbidites), (b) flocculation or sediment trapping by microbial mats, (c) settling from dilute suspension, (d) current flow prior to deposition, and (e) current flow during or after deposition. Other features are mudcracks, load casts, flame structures, bioturbation, graded rhythmites, fossil concentrations, cross-lamination, loop structures, and gradual variation in background sedimentation components, all of which carry information about conditions of sedimentation. We have examined these features in a collection of shale samples that range from Archean to Tertiary in age, and recorded them at a resolution ranging from 20 to 30 microns per pixel. The latter resolution is that of the microscopic imager (MI) in the MER rovers currently operating on Mars, and the former resolution is close to that of the hand lens imager (MAHLI) on Mars Science Lab. We can expect that by studying Martian mudstones at the hand-lens scale, they will reveal many sedimentary features useful for interpretation of sedimentary environments. These structures may reveal transport direction, flow energy and velocity, presence of surficial water bodies, and exposure. Although, no single feature can be used to pinpoint specific depositional conditions and environments, assemblages of sedimentary features can serve as environmental indicators.