Evolution of shallow-shelf mixed carbonate–siliciclastic systems in the rock record: icehouse vs. greenhouse stratal architectures

Shallow shelf mixed carbonate–siliciclastic environments are highly heterogeneous systems that vary substantially in facies, grain types, stacking patterns, and stratal architectures. Many factors influence the architecture and evolution of these systems, as dominant controls on both siliciclastic and carbonate systems are in play. Examples include sediment source material, input style (line or point-sourced), and rate of deposition; ocean chemistry, temperature, and salinity; currents and waves; and local climate. All of these variables make mixed carbonate–clastic systems particularly difficult to characterize. Further complicating the issue is the need to consider external forces impacting the systems, including tectonic activity, climate regime, and magnitude and frequency of glacioeustatic fluctuations.

Here, we compare two studies to highlight key differences in icehouse and greenhouse mixed systems. A subsurface study of the Pennsylvanian Upper Strawn Group, Eastern Shelf, Permian Basin, highlights extreme heterogeneity and evolution of a bathymetrically complex shelf adjacent to the subsiding Midland Basin in an icehouse setting characterized by rapid, high-amplitude sea level oscillations. Point-sourced clastics (deltas), ooid shoals, and robust carbonate banks form prominent discrete features and are found within close proximity to one another, but mixing occurs mostly at outer edges of adjacent depositional settings (e.g., quartz-rich ooid shoals on the fringes of delta lobes). In contrast, a mixed system preserved in the mid-Cretaceous Quitman Formation, Quitman Mountains, west Texas, is composed of much more thoroughly mixed sediments deposited along a shallow shelf adjacent to the subsiding Chihuahua Trough in a greenhouse climatic period. Facies are arranged in relatively flat-lying continuous belts, and include extensive, long-lived sandy oyster strandplains, mixed quartz–ooid shoals, and silty shallow subtidal shelves abundantly colonized by coral-rudist reefs and molluscan biostromes. Despite many similarities in depositional regime, stratal architectures are vastly different, which may be attributed to inherent differences in sedimentation styles in icehouse and greenhouse settings.