Stratigraphic Heterogeneity of a Holocene Ooid Tidal Sand Shoal: Lily Bank, Bahamas

Abstract

A central challenge in sedimentary geology is understanding three-dimensional architectural variability, and how it might be predicted. Ooid sand shoals, present in the stratigraphic record from Archean to recent, represent an economically important, but heterogeneous type of carbonate deposit. Whereas the processes influencing the deposition of ooid shoals are well examined and understood, the means by which distinct processes are recorded in the rock record are less constrained. The purpose of this thesis is to understand the relationship among processes, plan-view morphology and stratigraphic variability by examining Lily Bank, a modern tidally dominated Bahamian ooid shoal. Research focuses on two bar form end-members of Lily Bank: transverse shoulder bars oriented normal to flow and flood- and ebb-tide oriented parabolic bars. Results from integrating remote sensing imagery, high frequency seismic (Chirp) data and core characterization (sedimentary structures and granulometric analyses) reveal the stratigraphic record of geomorphic change. An irregular, but gently dipping surface acts as the base of the Holocene sequence. This lowest horizon (Z) is interpreted as the Top Pleistocene and shallows onto the platform. A lower unit (Unit A), up to 6 m thick, overlies this basal surface and is characterized as a poorly sorted gravelly muddy sand with abundant Halimeda, and almost no ooids. Unit A is capped by a transitional unit (Unit B) which shows an increase in ooids, better sorting, and larger grain sizes upwards. The uppermost unit (Unit B'), present only under the present-day bar forms, is a well-sorted ooid sand. Quantitative analysis reveals quantitative relations among units, geomorphic position, and granulometry. Sediments of Unit A are finer and less well sorted than those from Unit B. Likewise, the fraction of grains larger than 500 µm and sorting of sediments from crests of bar forms are clearly differentiated from those on the bar flanks. Collectively, these results provide more robust three-dimensional conceptual models of heterogeneity in ooid shoals.