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Resolving Mudrock Evolution with Silt Microtextures


Silt-grain composition and texture of mudrocks shed light on their sedimentary and diagenetic history. We investigated the silt microtexture of mudrocks by manually digitizing field-emission scanning electron microscopic (FE-SEM) images. The samples come from the Bexar member of the Pearsall formation (Late Cretaceous, Gulf section), and Barnett Shale (Mississippian, Fort-Worth Basin); a study of Marcellus microtextures is in progress. Sample selection was governed largely by the availability of samples with particulate microtextures; in our experience extensive carbonate cementation in samples with abundant carbonate grains makes quantifying silt microtextures difficult. The resulting characterizations include 500-1000 grains in any individual sample. Striking features of silt microtextures include the following. Firstly, there is a large range in silt percent, from 19-38%, though taken on its own the Barnett samples have lower variability from 20-27%. Secondly, the clast composition varies widely, with silicate-to-carbonate ratios ranging from 0.14 to 4.25. Thirdly, silt particles are only moderately elongate (aspect ratios from 0.49 to 0.66) but define very strong preferred orientations (vector strengths of 0.9). Lastly, the grain-size distributions, as a function of cumulative number vs. equivalent circular diameter (ECD), are exponential. These are Ae^-Bx type distributions, where B=0.364–0.724, and exponential tails fall off toward grain sizes of ∼10 μm. Similar distributions are observed for ECD-area distributions, though area distributions alone approach a power-law. In general, exponential grain-size distributions are commonly observed in fine-grained deposits. There is also no correlation between grain composition, silt abundance, fabric, and grain-size distribution. Therefore, post-depositional processes of grain crushing and dissolution were not extensive enough to greatly alter the primary grain-size distribution, shape, and potentially even the fabric. Moreover, transport processes were able to mobilize very fine silt grains. Altogether, we hypothesize that a great deal of the mudrock microtexture was established within the basin(s), despite both earlier transport histories and subsequent diagenesis.