Possible Sources of Dissolved Inorganic Carbon in the Formation of Middle and Upper Devonian Carbonate Concretions, Appalachian Basin

Abstract

Calcium carbonate concretions are common to the Middle and Upper Devonian shale succession of the Appalachian Basin. Geologic and microtextural evidence suggests that the authigenic carbonate formed at shallow burial depth, perhaps no more than a few tens of meters below the sediment-water interface, in a diagenetic environment resulting from the anaerobic oxidation of methane (AOM). Burial histories of the Middle Devonian Marcellus Shale and Upper Ordovician Utica Shale suggest that only the latter had generated a small amount of thermogenic methane by the time the Devonian shale succession started to accumulate. Thus, oxidized biogenic methane appears to have been the principal dissolved inorganic carbon source of the authigenic carbonate. However, modestly depleted δ13C values of concretions, from the Marcellus Shale upward through the Upper Devonian Dunkirk Shale, are well in excess of δ13C values of authgenic carbonate formed within a diagenetic environment induced by the anaerobic oxidation of biogenic methane. One explanation of this seemingly incongruent relationship entails a combination of the prolonged oxidation of shallow biogenic methane mixed with methanogenic CO₂, both of which were sourced at the bottom of the Marcellus Shale. Alternatively, some volume of the methane inventory consumed by AOM within the Devonian shale succession may have originated within the Upper Ordovician Utica Shale. It is plausible that “ancient” biogenic methane was released from the Utica shale, either over an extended period of time or as a geologically rapid event, to the overlying sedimentary column. Residual biogenic methane that finally reached the accumulating Middle and Upper Devonian deposits would have been only modestly depleted in 13C due to a protracted oxidation history. The obvious shortcoming of a scenario involving the expulsion of methane from the Utica Shale into the Middle and Upper Devonian shale succession is the presence of such intervening units as the Silurian Lockport Dolomite and overlying Salina Formation salt deposits. However, transport of methane from the Utica could have been enhanced by Acadian foreland basin dynamics, including salt removal, reactivated basement faults and the formation of Acadian faults and related fractures.

AAPG Datapages/Search and Discovery Article #90216 ©2015 AAPG Annual Convention and Exhibition, Denver, CO., May 31 - June 3, 2015