Patch reefs off Bermuda

[摘要] Patch reefs living on the Bermuda Platform provide excellent models for detailed investigation of bioclastic sediment dispersion. Skeletal debris is shed from the reef top to form a wedge of reef flank sediments which angularly onlaps the reef mass at the uppermost reef flank and grades into lagoonal sediments in deeper water. Major avenuesof sediment transport are reef face channels, which connect sand channels of the reef top with upper sections of the reef flank. Distance of sediment transport off the reef top is small--less than 100 meters.Compositional and textural trends across patch reefs are interpreted using a working model based on substrate control, biofacies development, particle breakdown, and sediment transport by wave action. Sediment composition provides the basis for recognition of the following microfacies: sediment pockets, sand channels, reef face channels, reef flank, interreef lagoon, and open lagoon. These microfacies are transitional and, in the above order, displaythe following trends: from the reef top lagoonward, the abundance of Homotrema, coral, and red algae in the sediment increase and the abundance of Halimeda and Foraminifera (excluding Homotrema) decreases. Interreef lagoons are distinguished from open lagoons by a lower mollusc content in the sediment. From the reef top lagoonward, mean grain size generally decreases (with certain anomalies explainable by the patch reef model of sediment dispersion), sorting progresses from poor to very poor, and skewness becomes relatively higher (due to winnowing of fine particles and addition of coarse reef material on the reef top). All kurtosis values indicate non-normal particle-size distributions and support the observed bimodality of sedimentsassociated with patch reefs.Sand-size particles dominate sediments of patch reef facies. The absence of silt and clay is characteristic of the reef top. Gravel is nearly evenly distributed, owing in part, to the large Halimeda content of the sediments (25%-60%) which is significant in this size fraction.Grain size distributions were obtained from a combination of sieving, Emery settling tube, and hydrophotometer analyses. Owing to the effects of irregular particle shapes, Emery tube data must be adjusted if a combination with sieve data is made. A graphic adjustment of such data is made which provides good approximations for mean and standard deviation, while maintaining relative relationships for skewness and kurtosis. Textural trends are displayed more accurately and are more easily recognized when moment calculations are used rather than graphic determinations of sediment parameters.Thus, distribution of bioclastic sediments from Recent Bermudan patch reefs takes place on a small, local scaleto form reef flank sediments, results in recognizable sediment microfacies, and is explainable by a sediment dispersion model.

[发布机构] Rice University