Burial environment, diagenesis, mineralogy, and MG & SR contents of skeletal carbonates in the Buckhorn asphalt of Middle Pennsylvanian age, Arbuckle Mountains, Oklahoma

Abstract

The Buckhorn asphalt quarry in the northern part of the Arbuckle Mountains, Oklahoma, contains chemically well preserved marine invertebrate fossils of Middle Pennsylvanian age. The main purpose of this investigation was to study the chemistry of these fossils, and at the same time analyze the geologic setting of the deposits containing the fossils. Due to early sealing by oil, many of the shells still have the original aragonite and nacreous luster. The oil prevented the grainstones from becoming well cemented and allowed them to become compacted during the Arbuckle orogeny. Deformation related to this orogeny probably caused the devolatization of the oil and, hence, conversion into asphalt. Most of the fossils occur as fragments in several skeletal debris grainstones which are shallow, turbulent water inferred channel deposits. The ancient shoreline was only six miles to the northeast. Underlying calcareous sponge spicule-echinoderm-brachiopod-fusulinid mudstones and wackestones were deposited in less turbulent water environments. Mineralogical data were obtained by means of x-ray diffraction, infrared spectroscopy, and chemical staining. Quantitative analyses of the elemental constituents for various shell layers were obtained with an electron microprobe. The original mineralogy of skeletal carbonates was established for the first time in the following. Aragonite occurs in the inner layers of the gastropods Naticopsis wortheni and Trachydomia whitei, whereas their outer layers consist of calcite. Aragonite also occurs in the shell walls of the scaphopod Plagioglypta ? sp., the coiled nautiloids Metacoceras cornutum, and Domatoceras sp., and the ammonoids Pseudoparalegoceras sp. and Wellerites mohri ?. The orthocone nautiloid Orthoceras unicamera has aragonitic shell walls, septa, and cameral deposits. Skeletons of the foraminifera Globivalulina sp. and Wedekinellina ? sp., the presumed tabulate Chaetetes cf. favosus, the cryptostome bryozoans Penniretepora ? sp. and Streblotrypa ? sp., the brachiopod Anthracospirifer opimus, four ostracodes, and the spines of the echinoid Archaeocidaris megastyla ? consist of calcite. As reported by others, calcitic outer layers and inner aragonitic layers were also found in Bellerophon (Bellerophon) sp., Straparollus (Euomphalus) sp., and Chaenocardia ovata. The shell walls and cameral deposits of Pseudorthoceras knoxense also are aragonitic. Previously unreported moderately high Mg contents occur in the calcite of Chaetetes cf. favosus. The spines of Archaeocidaris megastyla ? are inferred to have been originally Mg-calcite. Diagenetic effects were detected in the skeletal carbonates. The amount of replacement calcite and degree of obliteration of shell microarchitecture in the skeletal aragonites increase with decreasing asphalt content. Asphalt-impregnated skeletal calcites contain more Mg and usually less Sr than corresponding nonasphalt-impregnated specimens. Data obtained on the Mg and Sr concentrations of the best preserved specimens indicate the following. The Sr/Ca ratio for the coiled nautiloid Metacoceras cornutum is similar to that for the modern-day Nautilus sp. The Mg contents in the calcites of the foraminifera, bivalve, and ostracodes are similar to those in related Recent forms. The shell walls of the extinct orthocone nautiloids have lower Sr contents relative to the cameral deposits. These large differences are apparently due to the vital effect of the organism.