Compositional Classification for Fine-Grained Sediments and Sedimentary Rocks: Foundation for Bulk Rock Property Prediction

Abstract

In fine-grained sediments and rocks (>50 percent weight or volume of particles less than 62.5 μm) the primary grain assemblage reflects grain generation processes at deposition and is also an important control on the evolution of bulk rock properties in diagenesis. As in classifications for sandstones and limestones, the primary grain assemblage is a practical basis for classification of fine-grained materials. Tarl (terrigenous-argillaceous) refers to a grain assemblage with >75 percent of particles of extrabasinal derivation, including grains derived from continental weathering and also volcanogenic debris. Carl (calcareous-argillaceous) contains <75 percent of extrabasinal particles and among intrabasinal grains has a preponderance of biogenic carbonate particles including carbonate aggregates. Sarl (siliceous-argillaceous) contains <75 percent of extrabasinal particles and has a preponderance of biogenic siliceous particles over carbonate grains. These classes separate sediments of distinct depositional settings and contrasting organic matter content and minor grain types. Tarls dominate in thick mudrock successions characterized by high rates of sediment accumulation and typically contain little organic matter, much of it terrestrial. Carls and sarls are generally associated with thinner successions. The slower rates of accumulation for carls and sarls tend to favor generation of intrabasinal particles such as sediment aggregates (intraclasts, pellets, agglutinated allochems, etc.) and phosphatic debris. If organic-rich, carls and sarls tend to contain organic matter that originated in the water column. In the subsurface, tarls are relatively unreactive and only manifest significant reaction of the grain assemblage at elevated temperatures (>80° C). Under ordinary geothermal gradients tarls tend to remain unconsolidated until approximately 2 km of burial or more. In contrast carls and sarls contain chemically unstable grain assemblages (including labile organic matter) prone to react with pore fluids early in burial. Reactive grain assemblages in carls and sarls cause cementation and the generation of brittle rock properties relatively early in the burial history. Classification based on the grain assemblage is only the beginning of a complete rock description, but constitutes a valuable foundation for placing samples into the larger stratigraphic context and for making predictions about the post-depositional evolution of bulk rock properties.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015