Quantitative interrogation of a fossilized carbonate sand body – The Pleistocene Miami oolite of South Florida

Abstract

Exposures of the Pleistocene Miami oolite in South Florida provide excellent examples of preserved primary sedimentary features and subsequent diagenetic changes of a ‘fossilized’ ooid sand body that has been subaerially exposed in a tropical climate since its deposition during the last interglacial highstand – Marine Isotope Stage 5e. Using a robust airborne light detection and ranging digital terrain model and select outcrops, a detailed analysis of the morphologies and dimensions of the different portions of the Miami oolite was conducted. The sand body, extending 95 km north to south and approximately 15 km wide, consists of shoals (or bars) separated by tidal channels and is partly bounded on the ocean‐facing side by a prograding barrier bar that collectively cover an area of approximately 1000 sq. km. Results of the quantitative interrogation of the Miami oolite are compared with those of previously published work from modern sand bodies on Great Bahama Bank including the Exumas, Schooners Cays and Tongue of the Ocean. Digital terrain models developed for these modern sand bodies on the Great Bahama Bank and the ancient Miami oolite were used as the basis for the definition of shoals, shoal crests (peaks) and channels which were examined with respect to their size, shape and orientation. The Exumas sand body is the best morphometric match when analysing the bars and the channels, and the Joulter Cays area is an analogue for the formation of the barrier bar and for burrow reworking of cross‐bedding within stabilized sand shoals. The Exumas is a particularly compelling analogue for the Miami oolite with respect to length and overall visual comparison between the morphologies of the sand bodies, shoal (or bar) shape, number of tidal channels, channel length and width, and high areas parallel to the strike trend of the sand body (islands in the Exumas and possible islands as part of the Miami oolite barrier bar). Despite ca 115 kyr of subaerial exposure and meteoric diagenesis including karst, the morphology of the Miami oolite is still relatively intact to the point where it can be easily analysed and meaningful comparisons to modern systems can be made. The Miami oolite serves as a key reference example for comparison to Holocene sand units in the Bahamas – it validates the concept of comparative sedimentology and in particular emphasizes how results from the modern can improve the interpretation of a fossilized example.

Lessons from a modern carbonate sandbody – A personal experience of comparative sedimentology

“…Certainly, the upward coarsening and shallowing succession documented for the modern section in the Joulters area is common in ancient settings and is one of the several types of shallowing‐upward sequences discussed by James (). The lateral facies variability bears also the striking similarities to that recognized in Pleistocene limestones of South Florida (Evans, ; Halley, Shinn, Hudson, & Lidz, ; Purkis & Harris, ; Usdan, ) and the Bahamas (Beach & Ginsburg, ; Garrett & Gould, ) as well as numerous ancient subsurface examples (Bebout & Schatzinger, ; Major et al., ; Ottmann et al., ; Smosna, ). Halley et al.…”

“…Certainly, the upward coarsening and shallowing succession documented for the modern section in the Joulters area is common in ancient settings and is one of the several types of shallowing-upward sequences discussed by James (1979). The lateral facies variability bears also the striking similarities to that recognized in Pleistocene limestones of South Florida (Evans, 1984;Halley, Shinn, Hudson, & Lidz, 1977;Purkis & Harris, 2017;Usdan, 2014) and the Bahamas (Beach & Ginsburg, 1980;Garrett & Gould, 1984) as well as numerous ancient subsurface examples (Bebout & Schatzinger, 1978;Major et al, 1996;Ottmann et al, 1973;Smosna, 1984). Halley et al (1977) equated the shoal and channel portion of the Miami oolite in South Florida to modern Bahamian tidalbar belts including the Joulters example, and further suggested Joulters as an early-stage analog for the barrier bar portion of the Miami oolite.…”

“…The resulting accumulation—a narrow belt of ooid grainstone bordering a significantly wider belt of ooid packstone that becomes increasingly muddy with depth—is one that is observed in the geological record. Facies variability documented for the Joulters sandbody has served as a direct analog for guiding interpretations of outcrops and numerous subsurface examples where grainstones transition to packstones in a platformward direction (Bebout & Schatzinger, ; Harris et al., ; Major et al., ; Ottmann, Keyes, & Zeigler, ; Purkis & Harris, ; Smosna, ; Smosna & Koehler, ).…”

“…Ooid sands are reworked through substantial burrowing to become a widespread and thick layer of muddy ooid sands (packstone). Ooid sand islands develop locally in the sand flat, but the largest and highest-relief islandsthe Joulter Caysform along the seaward side of the sand flat as a part of the mobile fringe of remaining high-energy and ooid accumulation to packstones in a platformward direction (Bebout & Schatzinger, 1978;Harris et al, 1993;Major et al, 1996;Ottmann, Keyes, & Zeigler, 1973;Purkis & Harris, 2017;Smosna, 1984;Smosna & Koehler, 1993).…”

Impact of facies and diagenetic variability on permeability and fluid flow in an oolitic grainstone—Pleistocene Miami Oolite

“…Exposures of the Miami Oolite (MO) in the vicinity of Miami, Florida, USA, which are equivalent to the oolitic facies of the Miami Limestone of Hoffmeister et al (), provide excellent examples of preserved primary sedimentary features and subsequent diagenetic changes of a relatively young, yet ‘fossilized’, carbonate sandbody. This Pleistocene‐age formation, deposited approximately 120 kyr bp during the last interglacial highstand (Marine Isotope Stage 5e or MIS 5e), serves as a reference example for comparison to Holocene sand units in the Bahamas (Purkis and Harris, ), and more importantly, to subsurface hydrocarbon reservoir and aquifer examples in the geological record. Continued interest in modern and outcrop analogues for carbonate sand reservoirs, like the MO, is warranted based on the substantial number of carbonate hydrocarbon reservoirs that produce from grainstones and packstones (Wilson, ; Harris, ; Roehl and Choquette, ; Keith and Zuppann, ; Harris and Weber, ).…”