Cratonic sequence stratigraphy: Advances from analysis of mixed carbonate-siliciclastic successions.

摘要

The following series of papers addresses the need for a sequence stratigraphic model specifically designed for cratonic mixed carbonate-siliciclastic successions. Case studies are provided primarily from the mixed carbonate-siliciclastic strata of the Lexington Limestone of Kentucky. These studies explore the sub-regional and regional distribution of decameter-scale couplets composed of clean skeletal grainstone and argillaceous limestones interbedded with shales. Analysis of six couplets that make up the Lexington Limestone along a basin profile reveals that both parts of decameter-scale couplets are widely traceable, though each undergoes a gradual lateral facies change. Subsequent investigation reveals that these six couplets are regionally traceable along strike, showing particularly good similarity to age equivalent strata in New York. More detailed studies are also provided that focus on different aspects of the couplets to help reinforce their sequence stratigraphic significance. The uppermost skeletal grainstone unit of the Lexington Limestone and basal portion of the overlying Kope Formation are analyzed in great detail, incorporating stratigraphic correlation of individual beds between closely spaced exposures, sedimentology of condensed beds and discontinuity surfaces, and faunal and taphonomic gradient analysis of limestones within this interval. The data generated suggest that this grainstone-rich succession represents a deepening-upward succession formed during sea level rise, though with slightly varying degrees of influx of argillaceous sediments (lowstand, early transgression, and late transgression, respectively). Additional case-studies focus on the contact at the base of the grainstone-dominated half of the couplet. This contact, contrary to previous studies, is almost always sharp and erosional. In fact, detailed analysis reveals two closely spaced erosion surfaces, one at the contact of the two halves of the couplet (forced regression surface), typically overlain by argillaceous calcarenite (falling stage systems tract), and one slightly higher (sequence boundary) overlain by more massive grainstones (lowstand, etc.). Combination of the case-study data with the literature of discontinuity surfaces and condensed beds allows for the formation of a general sequence stratigraphic framework for foreland basins. Finally, a unified model for foreland basin sequence stratigraphy is presented by integrating knowledge of the carbonate margin of middle Paleozoic foreland basins with well-established models concerned with the siliciclastic margin.