AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
A Global Survey of the Geometry and Sedimentology of Holocene Shelf-Margin Reefs and Sandy Backreef Aprons
(1) Dept of Geology, University of Kansas, Lawrence, KS.
(2) Schlumberger-Doll Research, Cambridge, MA.
(3) National Autonomous University of Mexico, Sisal, Mexico.
Shelf-margin reefs and sandy backreef aprons represent important elements in many hydrocarbon systems. Although it is obvious that reefs and backreef aprons are most common near the margins of isolated platforms, their sizes and the environmental controls on their distribution and sedimentology are much less well quantitatively constrained. To better constrain possible variability in ancient analogs, systematic remote sensing analysis of 65 isolated platforms and field observations from nine platforms provide quantitative insights into the mode, magnitude and controls on local, regional, and global patterns in the sedimentology and geometry of Holocene shelf- margin systems. Results indicate: 1) Reefs cover from < 1 to 55% of platform tops (mean = 20%), whereas backreef aprons represent between 0 and 65% of platform tops (mean = 20%), whereas islands and platform interior facies form the remainder. 2) The relative abundance of reefs (% of platform top) is inversely proportional to platform size; the abundance of backreef aprons do not show a comparable trend. 3) Reefs and backreef aprons from all platforms combined (and many individual platforms) have a lognormal width-frequency distribution. 4) Margins facing the largest waves (waveward) have wider reefs and reef aprons; these margins may, or may not, face the dominant winds (windward). Reefs and reef aprons on both windward and waveward margins are systematically wider than along other margins, however. 5) Sediments on many sandy backreef aprons include systematic lagoonward fining and an increase in abundance of rounded and abraided grains, both related to waning energy. Areas more than a few 1000 m inboard from the margin include abundant fine sand and mud (in many cases exceeding 30%), except for immediately next to patch reefs, although tidally dominated platforms have less mud. 6) Grain types vary with changes in ocean chemistry. Beyond corals and calcareous algae, ooids and peloids are found exclusively in areas with an elevated carbonate saturation state, and heterozoan (foram, mollusc, red algae) associations can occur in equatorial upwelling regions. Collectively, the comparative analysis illustrates actualistic, process-based facies models for the nature and controls on variability of shelf margin systems, within and among isolated platforms. The data and insights could prove useful for development of more realistic and accurate reservoir models of ancient analogs.