AAPG ACE 2018

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Statistical Pattern Analysis of Dolines in the Pleistocene Miami Oolite of South Florida

Abstract

The Miami oolite displays the preserved morphology of a fossilized ooid sand body, despite being subaerially exposed in a tropical climate since its deposition during the last interglacial highstand – Marine Isotope Stage 5e. But a bare-earth LiDAR DTM also shows a surface that is locally modified by karst features, primarily shallow karst depressions (dolines) and a few shallow stratiform caves. Detailed analysis can perhaps aid prediction of reservoir character in analogous karsted oolite reservoirs.

The highest density of dolines occur in the most seaward portion of the and body – the strike-oriented barrier bar. Here, 82% of dolines occupy troughs formed between prograded spit ridges. This distribution suggests the depositional topography of the barrier bar to exert a degree of control on the pattern of subaerial dissolution. Considering the full population of 735 dolines mapped in the Miami oolite, their lateral distribution is highly clustered, as would be expected by the fact that dolines are only recognized in the highs of the sand body (i.e. the barrier bar and lower-relief, dip-oriented shoals). Assessing the lateral distribution of the dolines across a 500 m grid reveals that their average separation distance on the barrier bar versus shoals to be statistically inseparable, but an important difference in patterning between these two provinces is observed. Dolines on the barrier bar are either uniformly disposed or clustered, while those on the shoals are randomly arranged. This result can be related back to the depositional topography of the barrier bar controlling dissolution, whereas in the subdued topography of the shoal system, the arrangement of dolines is less regular to the point of being statistically indistinguishable from random.

Both the areal extent of the dolines and their depth of dissolution are well-described by simple mathematical functions; the size-frequency distribution robustly follows an exponential distribution and the depth of penetration is normally distributed with a mean depth of 3 m above present-day sea level. There also exists a relationship such that the depth of the dolines increases as a function of their size. The average depth of the population of dolines is only 0.45 m, emphasizing the fact that very few dolines attain depths >1 m. Assuming an average denudation rate of ~0.01 m kyr-1, this distribution would suggest that the surficial dissolution is the product of the relatively long-lived exposure.