Decoding the origins and sources of clay minerals in the Upper Cretaceous Tununk Shale of south‐central Utah: Implications for the pursuit of climate and burial histories

Abstract

Clay minerals in fine-grained marine sedimentary successions are most commonly considered to be detrital in origin and have been used extensively by geologists as indicators of palaeoclimate conditions in the hinterland. Most of these previous studies, however, were not designed to address in depth the potential effects of mixing clay minerals from multiple sources and the formation of authigenic clay minerals during early diagenesis on the ultimately observed clay mineral assemblages of fine-grained marine sedimentary successions. Herein, clay minerals in shales and bentonites of the Tununk Shale Member in south-central Utah were examined through integrated X-ray diffraction and petrographic (scanning electron microscopy) analysis, to evaluate the various origins of clay minerals in this offshore mudstone succession. Clays in Tununk bentonites contain dominantly smectite (>80%) and a minor amount of kaolinite. Clays in Tununk shale samples consist dominantly of mixed-layer illite/ smectite with up to 45% illite-like layers, small amounts of kaolinite and mica, and in places trace amounts of chlorite. Clay minerals in Tununk shale samples occur in the following three forms: (a) in clay-dominated aggregates (i.e. smectite-dominated altered volcanic rock fragments and illite/smectite-dominated shale lithics); (b) in the fine-grained matrix (mostly illite/smectite and minor amounts of mica and kaolinite); and (c) in intergranular and intragranular pore spaces (authigenic smectite, kaolinite and chlorite). Possible sources for the mixed-layer illite/smectite in shales include (a) erosion of older smectite-bearing mudstone successions and (b) weathering of volcanic rocks or volcanic debris that had been deposited on land. Most of the kaolinite and chlorite in the Tununk Shale were precipitated as pore-filling cements, rather than having a terrigenous source (as weathering products). A comprehensive understanding of the multiple origins of clay minerals (e.g. terrigenous input, volcanic input, recycled sediments and diagenesis) in marine mudstone successions is critical when attempting to use clay mineral data for reconstructions of palaeoclimate and burial and thermal histories. | 173 LI et aL. How to cite this article: Li Z, Schieber J, Bish D. Decoding the origins and sources of clay minerals in the Upper Cretaceous Tununk Shale of south-central Utah: Implications for the pursuit of climate and burial histories. Depositional Rec. 2020;6:172-191.

Reply to the Discussion by Al‐Mufti on “On the origin and significance of composite particles in mudstones: Examples from the Cenomanian Dunvegan Formation” by Li et al. (2021), Sedimentology, 68, 737–754

“…They are the end result of the interaction between actual particles and currents of water or air. It is for that reason that, for example, carbonate classifications have an aspect that reflects depositional energy (textural maturity, Folk, 1959; amount of mud, Dunham, 1962), and why there are mudstones that by way of fluid dynamics started out as an accumulation of sand‐sized particles (Rust & Nanson, 1989; Sternberg et al ., 1999; Schieber et al ., 2010; Schieber, 2016; Laycock et al ., 2017; Li & Schieber, 2018; Schieber et al ., 2019; Li et al ., 2020, 2021). If that circumstance goes unrecognized when a seemingly simple mudstone is investigated, erroneous interpretations of depositional processes and ambient energy conditions of these rocks are likely to follow.…”

On the origin and significance of composite particles in mudstones: Examples from the Cenomanian Dunvegan Formation

“…3A and B), similar to the mineral composition and texture of volcanic ash after burial diagenesis in marine environments (i.e. volcanic glass altered into smectite during early diagenesis, then into illite during deeper burial; Li et al ., 2020). The original flux of most volcanic ash into the Dunvegan system was probably wind‐transported (Nadeau & Reynolds, 1981).…”

Fluvial and Marine Depositional Environments Determined From Groundwater Chemistry and Calculated Clay‐Cation Compositions

“…Few studies, however, have been done to characterize the clay distribution along the fluvial to marine transition zone in a coastal environment (e.g., Gugliotta et al., 2020). In addition, recent advances in understanding the origin and distribution of clay‐rich sediments indicate that clay properties, such as clay mineral content, assemblages, distribution, and depositional patterns, are too complex and generally a product of the source material, with insufficient correlation between specific clays and the depositional environment, to allow interpretation of environment based on clay properties alone (e.g., DeReuil & Birgenheier, 2019; Li & Schieber, 2018; Li et al., 2020). Other approaches show how the mineralogy of clays (not their cation composition), and their fraction of the sediment can be used as indicators of depositional environments to differentiate marine from non‐marine conditions (e.g., Gibbs, 1977; Griffin & Parrot, 1964; Lonnie, 1982), but this requires acquisition of sediment samples which are not always readily available and also have uncertainties in their interpretation (e.g., Knox et al., 2006).…”