Depositional Topography: Key Element of Stratigraphic Interpretation and Panacea for Log Correlation: Part 2: Icehouse and Greenhouse Systems

Charles Kerans and Scott W. Tinker: Bureau of Economic Geology, The University of Texas at Austin, Austin, Texas

Abstract

Global eustasy has always formed a critical underpinning of the concepts of sequence stratigraphy. An important aspect of global eustasy that has come into focus during the last decade is the secular nature of the high-frequency (=<400 Ky) Milankovitch-driven eustatic (and climatic) signal. Specifically for marginal marine and shallow marine systems, it is now recognized that classification into greenhouse, transitional, and icehouse systems provides an important element of predictability that can assist in the important task of recognizing and characterizing depositional topography for use in construction of stratigraphic frameworks.

Greenhouse systems with their high global eustatic sea levels, small-amplitude eustatic oscillations, and periodically highly stratified water columns, produce a distinctive stratigraphic pattern. The highest frequency signal (20-100 Ky) of cyclicity is dampened in the shallow-water bank-margin areas and becomes better represented in basin-floor and platform interior settings where climatic signals override physical sedimentary processes. This loss of high-frequency cycle signal in shelf-margin areas, combined with the low-amplitude eustatic range associated with intermediate-duration (400 Ky - 1 My) cycles, exacerbates the correlation problem. In the absence of fine-scale cycle-based time surfaces, the use of facies-topography form surfaces is a must to avoid overemphasizing lateral continuity of reservoir flow units, particularly in prograding foreshore-shoreface complexes. Examples of depositional dip profiles tied to grain size and depositional energy illustrate the necessary emphasis on this geomorphically driven interpretation approach.

Icehouse systems also require a clear understanding of rock-based depositional topography for accurate construction of stratigraphic frameworks. Extreme high-amplitude, high-frequency eustatic shifts associated with these systems leave steep, local depositional gradients (unfilled accommodation), many times erosionally enhanced. No simple depositional geomorphology rules may apply in these settings.