Little Ambergris Cay, a Case Study for Ooid Rich Island Development on the Turks and Caicos Carbonate Platform

Brown, Drew G.

Undergraduate Honors Thesis

Little Ambergris Cay, a Case Study for Ooid Rich Island Development on the Turks and Caicos Carbonate Platform Pubblico Deposited

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Citeable URL: https://scholar.colorado.edu/concern/undergraduate_honors_theses/8910jv661

Abstract

Little Ambergris Cay (LAC) within the Turks and Caicos is a useful field site to examine the accumulation of a carbonate island, within a typical carbonate platform environment, because it does not follow the typically accepted modes of island accretion. There are several methods used to describe carbonate island formation after Schlager (2003), who described carbonate “factories” in reference to the modern analogues of Florida and the Bahamas. The Turks and Caicos, and specifically Little Ambergris Cay, offers a field side that differs from the comparison analogues in sediment composition, energy flux, and Holocene development. Little Ambergris Cay is unusual in that it formed during a period of sea level rise (Figure 2, Toscano & Macintyre, 2003), and thus presents an opportunity for research on how modern carbonate platform environments accumulate through time. This paper presents an argument for eastward direction of island accumulation of Little Ambergris Cay. Based on the research of Trower et al. (2018), Dravis & Wanless (2008), and Schlager (2003), I interpret that Little Ambergris Cay formed via eastward accumulation based on the influence of wind-wave current energy as means of supplying ample sediment to a zone of accumulation; in other words, in this system, sediment supply outpaces accommodation space. To support this interpretation, I will present several sets of data that were derived from unlithified sediment cores collected from the interior of LAC as means to characterize and compare against representative environments. First, I performed thin section point-count analysis on loose sediments from a variety of depths in a representative collected core to identify how grain composition varied with depth. I then compared this point count data with a range of grain characteristics such as grain size, sorting, sphericity, etc., determined via Camsizer analysis, in order to determine the parameters that best described the changes in grain concentration as seen in the point count. Finally, by integrating plots of the grain size parameters determined in this paper vs. depth, stereoscope microscopy, and radiocarbon ages of core sediments, I compared representative facies of modern carbonate environments to determine a facies development through time of Little Ambergris Cay. I concluded that Little Ambergris Cay underwent a series of high to low energy facies changes, where early lithification on the south-western edge of the modern LAC allowed successive accumulation of incoming easterly ooids due to sediment entrapment, possibly due to microbial mat and bedrock influence. The incoming sediment grains from the east onto an early lithified fragment to the west allowed eastward accumulation, where facies changed from an ancient shoal to a modern island. This paper furthers the research of modern carbonate analogues and their interpreted growth patterns. LAC offers a unique environment that does not follow typical carbonate island development, and by providing a case study of wind-wave influenced island accretion, sectors of geologic interest have the opportunity to interpret unique climatic and chemical factors. The petroleum unconventional subsurface carbonate play search is greatly related to LAC, as grainstones offer potential reservoirs (Dravis & Wanless, 2008). The geochemical field may have interest in the accumulation of LAC, as specific seawater concentrations play a role in the uniquely high concentrations of ooids found on the Caicos Platform. The geobiological field may be interested in LAC, as the relatively low abundance of microbes and biotic influence differs greatly from the other modern analogues for carbonate platform development.

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