Numerical forward modelling of peritidal carbonate parasequence development: implications for outcrop interpretation

Abstract

The mechanisms responsible for formation of peritidal parasequences have been a focus of debate between proponents of contrasting autocyclic and allocyclic models. To contribute to this debate a three‐dimensional numerical forward model of carbonate production, transport and deposition has been developed. Shallowing‐upward parasequences are produced in the model via carbonate island formation and progradation, with an element of self‐organization, and no external forcing. These autocyclic parasequences have characteristics comparable with peritidal parasequences observed in outcrop. Modelled parasequence thickness and duration depend primarily on subsidence rate and sediment transport rate, illustrating the significance of sediment flux in formation of peritidal parasequences. Adding an element of stochastic variation of sediment transport rate and transport path leads to formation of nonuniform‐thickness parasequences that generate Fischer plots showing apparent hierarchies similar to those often interpreted as evidence of eustatic forcing. The model results do not rule out allocylic mechanisms, but suggest that shoreline and island progradation are also plausible mechanisms to create variable‐thickness, shallowing‐upward peritidal parasequences and should be considered in interpretations of such strata.

Geomorphic evolution of a storm-dominated carbonate ramp (c. 549 Ma), Nama Group, Namibia

“…Flemings & Grotzinger, 1996) and rules-based (e.g. Drummond & Dugan, 1999;Burgess, Wright & Emery, 2001) process models.…”

“…Evaluation of the mechanisms by which geomorphic variability is propagated across spatial and temporal scales has become a growing focus of research in the earth sciences (Howard, 1997;Paola et al 2001;Dietrich et al 2003) with potential application to carbonate depositional systems (Drummond & Dugan, 1999;Burgess, 2001;Burgess, Wright, & Emery, 2001;Rankey, 2002;Burgess & Wright, 2003). Pattern formation in these systems at scale 'n' characteristically results from interactions between intrinsic component subsystems at scale 'n − 1' (self organization), perhaps modulated by processes at scale 'n + 1' (extrinsic forcing).…”

Sensitive dependence, divergence and unpredictable behaviour in a stratigraphic forward model of a carbonate system

“…In all other respects, the model is unchanged from that described in Burgess et al (2001) and Burgess & Wright (2003); steady subsidence, carbonate production, and landward sediment transport create progradational shorelines and islands that, in turn, create shallowing-upward carbonate parasequences in a manner similar to that initially described by Ginsburg (1971). Although no tidal processes are calculated in the model, a tidal range of 1 m is assumed in order to apply supratidal, intertidal and subtidal classifications to deposited sediments and thus to simulate peritidal strata.…”

“…In contrast to previous versions of the model described in Burgess et al (2001) and Burgess & Wright (2003), spatial distribution of carbonate production in the model is controlled by a production mosaic calculated using deterministic cellular automata. A cellular automaton is a grid of values with simple rules specifying how the value at each point is calculated for the next time-step according to the value of adjacent points (e.g.…”

“…Maximum rate modifier 0 0.2 0.4 0.6 0.8 0 Model results presented here were generated with a numerical stratigraphic forward model modified from that described in Burgess et al (2001) and Burgess & Wright (2003). The model is a representation of combined processes of carbonate sediment production and transport.…”