Carbonate Sequences as Complex Systems: Geochemically Influenced Sediment Supply Issues
Authors: Kerans Charlie 
Published: November 2019

Outcrop, core, log, and seismic data from carbonate sequences ranging in age from Proterozoic through Pleistocene have led to the understanding that the carbonate stratigraphic record is best understood as a complex system: i.e., any system featuring a large number of interacting components (agents, processes, etc.) whose aggregate activity is nonlinear (not derivable from the summations of the activity of individual components) and typically exhibits hierarchical self-organization under selective pressures.”

(www.informatics.indiana.edu/rocha/publications/complex/csm.html).

Stratigraphers typically work with a limited set of regime variables (sea level, subsidence, sediment supply, climate) in order to predict patterns of accumulation from seismic scale down to grain-size and pore networks, but typically wind up short. Deviations from what would be predicted using the already complex multivariate parameters above are the norm not the exception. Clearly a broader spectrum of inputs, short-lived and long-term, periodic, and chaotic is at play and demand consideration. Here I focus on three examples of geochemically influenced sediment supply patterns that fundamentally alter sequence architecture but that are not easily predicted from standard consideration of the A/S equation.

Permian mixed siliciclastic-carbonate sequences of the Permian Basin have served as a testing ground for carbonate sequence stratigraphy since the pioneering publications by Exxon in the mid 80s. RCRL began research in these outcrops in 1987 starting with San Andres ramps, Grayburg mixed siliciclastic-carbonate shelves, and more recently complexly faulted Capitan reef-rimmed profiles. A framework has been developed in the outcrop that captures much of the system and serves as an important guide for deciphering the Delaware and Midland basin patterns as well as the Northwest Shelf/Central Basin Platform record. Though A/S is a good high-level predictor of sequence development, facies substitution and evolution across ramp/rim transitions requires an understanding of basin geochemistry and slope stability at a range of scales. Ignoring these controls hampers prediction of such fundamental attributes as depositional profiles, reservoir facies distributions, and of reef morphology and evolution.

Greenhouse (Cretaceous/Jurassic) carbonate sequences of the Gulf of Mexico illustrate another challenge to standard A/S-driven patterns. Order-of-magnitude perturbations in carbonate factory rates are seen during oceanic anoxic events with their attendant decrease in oxygenation and potential ocean acidification. These drastic impacts on the carbonate factory cause shifts in accumulation patterns that are not simply linked to base-level. Because geochemical forcing varies significantly within a basin and between basins as driven by oceanographic effects, eustatic signals typically do not produce regionally mappable and predictable sequence frameworks.

In young highly constrained carbonate sequences of the mid-late Pleistocene the impact appears even more dramatic, as two to four times increases in depositional rates can be shown to occur within a 2-4 ky time scale. Such “explosions” of ooid facies as shown in the Caribbean are likely analogous to the “overshoots” tied to oceanic anoxic events like the Toarcian. Currently our more widely applied sequence models do not predict these complex system responses. These deviations should excite and challenge researchers dedicated to unraveling carbonate sequence stratigraphy. Rather than becoming passé or irrelevant, carbonate sequence stratigraphy is an essential first step in constraining known parameters, allowing focus to shift to critical but less well understood signals.