Abstract: Recognition of Subaerial Exposure in Lower Paleozoic Carbonates: New Insights From Discontinuity Surfaces in Ordovician Oncolitic Limestones, Southern Nevada

Fredrick D. Siewers, Phillip A. Sandberg

Over 20 discontinuity surfaces crop out in an 85 m interval of homogeneous Middle Ordovician (Whiterockian) oncolitic limestone, Arrow Canyon Range, southern Nevada. Many of the surfaces exhibit intertidal paleokarst features marking periodic exposure events during oncolite deposition. Other predominantly planar surfaces lack clear exposure indicators and are difficult to interpret due to the lithologic homogeneity of the succession. Detailed petrographic and geochemical analysis of surface-adjacent lithologies supports morphologic indicators of exposure and shows that some planar surfaces resembling submarine hardgrounds also define exposure events.

Three "cement types" are recognized and interpreted as products of early near-surface to shallow burial diagenesis in subtidal and intertidal settings. Stable isotopic compositions of relatively unaltered cements and micrites (av. ^dgr¹⁸O=-7.17, ^dgr¹³C=-1.72) are close to reported coeval brachiopod compositions (av. ^dgr¹⁸O=-6.5, ^dgr¹³C=-O.53)(Chazyan, Valcour Formation, NY), suggesting precipitation from marine to near-marine waters. Paleokarst surfaces are characterized by slight depletions (less than, or equal to 1^pmil) in mean ^dgr¹⁸O with respect to immediately overlying lithologies as are some planar discontinuities higher in the succession. These data reflect the influence of meteoric and/or mixing zone waters during exp sure. Planar discontinuities bound by lithologies lacking similar isotopic shifts are interpreted as entirely marine hardgrounds.

These Ordovician discontinuity surfaces lack the paleosols and significant ^dgr¹³C excursions commonly associated with subaerially-exposed post-Silurian limestones. Nevertheless, the observed ^dgr¹⁸O excursions are distinctive and, where associated with discontinuities in other lower Paleozoic sections, may permit the recognition of past exposure events.

AAPG Search and Discovery Article #90986©1994 AAPG Annual Convention, Denver, Colorado, June 12-15, 1994