Ichnologically Influenced Porosity on a Holocene Isolated Platform

Mary, Michelle; Rankey, Eugene C.; Uriam, Tion

Beyond aiding grain diminution, boring organisms create macro- and micro-porosity. This porosity can be preserved, occluded, or facilitate the subsequent wholesale diagenetic alteration of grains. Although several studies have illustrated styles and intensity of bioerosion at individual locations in modern systems, this information remains poorly constrained at the larger platform scale, or in reservoir analogs in which it has been described (e.g., Arab-D reservoir, Ghawar; Mauddud reservoir, Bahrain). To develop a conceptual model for the nature, distribution and magnitude of borings in ancient analogs, this study evaluates and quantifies the nature and controls of ichnologically produced intra-particle porosity, and how it varies within and among shallow Holocene carbonate systems.

To capture a range of ocean chemical (pH, alkalinity, saturation state, and nutrient levels) and geomorphic variability, transects on Aranuka Atoll, Kiribati sample differences in orientation (windward, leeward, oblique) and bounding water masses (ocean-facing, lagoon-facing). On each transect, field measurements of pH, temperature, and alkalinity were recorded and experimental blocks were placed across the platform margin (reef and backreef sand apron subenvironments). After a year, blocks were retrieved and petrographically examined to capture rates of bioerosion. On Aranuka, average linear bioerosion rates (per block) vary from 0.13-0.59 mm/yr (n=6, maximum local rate 4.07 mm/yr and minimum 0.03 mm/yr), with the highest rates in the waveward portion of the platform. The average linear bioerosion rate in Aranuka is positively correlated with distance from the platform margin (R2 = 0.81), but includes only ambiguous relations with water chemistry. Whereas the metric of distance encompasses multiple variables, its correlation with bioerosion rate may provide a useful model for predicting ichnologically influenced porosity trends in ancient analogs.

AAPG Search and Discovery Article #90163©2013AAPG 2013 Annual Convention and Exhibition, Pittsburgh, Pennsylvania, May 19-22, 2013