ABSTRACT

The macroscopic features that characterize subaerial exposure surfaces in carbonates are well known, but their significance has not been quantitatively evaluated. This study presents such an analysis in the lower Oligocene Suwannee Limestone of west-central Florida. Eleven cores were point counted on a foot-by-foot basis for the abundance of caliche, rhizoliths, karst breccia, open vugs, infiltrated sediment, fractures, and pedogenic alteration. These features occur at and below intraformational exposure surfaces, which represent hiatuses estimated at 10⁴ to 10⁵ years, and an uppermost sequence-bounding unconformity representing 0.5 Myr, as revealed by Sr-isotope data.

Statistical analyses of the point-count data reveal only a few significant relationships. (1) The hierarchy of exposure surfaces, and by inference duration of exposure, is differentiated only at a marginally significant level by sediment-filled vugs preferentially associated with the sequence boundary. Duration of exposure did not have a significant impact on the relative abundance of all other features. (2) Proximity (< 5 ft; 1.5 m) to any exposure surface is indicated only by rhizoliths, caliche, and pedogenic alteration, whereas karst breccia is preferentially found distal (> 5 ft) to both types of surfaces. Fractures, open vugs, and infiltrated sediment show no proximal or distal preference for either type of surface. (3) Depositional texture has no statistically significant affect on the presence or abundance of the exposure features, with the exception that rhizoliths and open vugs are preferentially more abundant in packstones relative to grainstones. This is interpreted to be the result of a soil-moisture effect.

Factor analysis defines four factors that explain 46% to 52% of the total variance in the abundance data relative to the sequence boundary and the intraformational surfaces, respectively. The loading of each exposure feature on each factor is the same with respect to both types of surfaces, which is further evidence that the abundance of exposure features is independent of duration of exposure. Factor 1 is interpreted to be the amplitude of base-level changes and controls the abundance of karst breccia. Factor 2 is interpreted to be abundance of vegetation and relates to the abundance of rhizoliths and fractures. Factor 3 is interpreted to be a combination of soil-zone PCO₂ and the availability of water and affects the abundance of pedogenic overprinting, caliche, and open vugs. Factor 4 is stratigraphic proximity to the sequence boundary, which controls the presence of sediment-filled voids. The amount of uncorrelated unique variance associated with infiltrated sediments, pedogenic overprinting, caliche, and open vugs is large (> 60%), meaning that feature abundance is also influenced by other unidentified site-specific factors.

These results demonstrate that quantifying the abundance of macroscopic subaerial exposure features in limestones has the potential to yield more insight into the significance of those features than a mere qualitative assessment. This is particularly true when assessing the potential role of the many variables that can affect the development of these features.