Stony Mountain (Ordovician) and Interlake (Silurian) Facies Analogs of Recent Low-Energy Marine and Subaerial Carbonates, Bahamas

During Late Ordovician and Silurian times, epeiric carbonate sediments were deposited over a negative metastable lineament of the western Williston basin. These sediments are now the Stony Mountain Formation and Interlake Formation dolomites. The original sedimentary and early diagenetic fabrics are still preserved and, as a group, characterize a complete cycle of low-energy carbonate sedimentation. They also determine the distribution and internal geometry of interparticle and intercrystalline porosity. Finally, these fabrics are analogous with modern platform infratidal (shallow subtidal), intertidal, supratidal, and subaerial diagenetic terrane deposits of the Bahama Islands. Minor transgressive deposits punctuate the sequence but are important mainly in that they reinitiated offlap deposition. Ultimately the basin became fully emergent and the sediments underwent the last but most widespread penecontemporaneous dolomitization, solution alteration, and evaporite deposition similar to the present-day sebkha-wadi complex (supratidal) of the Trucial Coast, Persian Gulf. An extensive period of subaerial exposure ensued, linking the positive attitude of the Williston basin with that of the continent until Middle Devonian time. During emergence a microkarst developed over minor positive structural elements. As a consequence some reservoir rock of the uppermost Interlake was destroyed by superposed stream erosion. A generalized working model of low-energy facies relations is presented showing the proposed environment of offlap deposition and some of the fabric types in which producible oil is found. These fabrics are pelleted, laminated, and burrowed “lime mud” and silt, algal mats, and stromatolites, flat-pebble breccia, epigenetic (non-tectonic) fragmental and solution breccia, and a few cut-and-fill structures. Leaching of fossils and anhydrite in certain places has accentuated and improved pore structures. Infratidal and intertidal facies deposits, including their storm and spring high-tide equivalents onshore, are the most widespread and consistent, but least diagnostic, of facies environments. Supratidal and subaerial diagenetic terrane remnants on the other hand are most diagnostic, having several distinctive microfacies fabrics. Because these fabrics commonly grade from one to the other in short distances, they are difficult to evaluate as prospective major oil-bearing units.