David A. Budd¹, Stephanie B. Gaswirth¹

(1) University of Colorado, Boulder, CO

ABSTRACT: Quantification of Macroscopic Diagenetic Subaerial Exposure Features in Carbonate Rocks

The diagenetic features that characterize subaerial exposure surfaces in carbonates are well known, but their significance has never been quantitatively evaluated. This project developed and tested a point-counting method for performing such an analysis on the face of slabbed core in a case study of the Oligocene Suwannee Formation. It is assumed that paleoclimate, depositional mineralogy, and vegetation type were constant during Suwannee deposition. The results are thus evaluated as a function of regional paleotopography and duration of exposure (intraformational hiatuses of 104 to 105 years versus a sequence boundary representing >106 years of exposure). Twelve cores were point-counted on a foot-by-foot basis, each exhibiting one to eight different exposure surfaces. The macroscopic features noted were caliche, rhizoliths, reddish stain, collapse breccia, pedobreccia, vugs, and infiltrated siliciclastic sediment.

Although differences in the abundance of features occur throughout and between wells, only a few statistically significant spatial and temporal patterns are present. (1) Open vugs are preferentially associated with intraformational surfaces whereas vugs filled with siliciclastic sediment from the overlying formation are preferentially associated with the sequence boundary. (2) Pedobreccia is preferentially absent from all exposure surfaces on regional paleolows. (3) Collapse breccia is preferentially found proximal to intraformational surfaces, which is probably a coincidence reflective of paleocave formation at multiple horizons below the sequence boundary. (4) Collapse breccias at the sequence boundary preferentially occur on regional paleohighs.

There are no criteria whose differential abundance defines short versus long-lived exposures, nor any that define regional paleohighs from paleolows. In fact, the insights gained from the rigorous quantification are basically no different from those gained by a simple notation of presence or absence. With respect to the test case, there thus appears to be few advantages to a quantitative analysis of exposure features in carbonate rocks.

AAPG Search and Discovery Article #90906©2001 AAPG Annual Convention, Denver, Colorado