2002 Denver Annual Meeting (October 27-30, 2002)

Paper No. 25
Presentation Time: 1:30 PM-5:30 PM

CENTIMETER-SCALE CHARACTERIZATION OF EVENT BEDS WITHIN THE ALEXANDRIA SUBMEMBER OF THE KOPE FORMATION (UPPER ORDOVICIAN, EDENIAN) IN THE CINCINNATI, OH, NORTHERN KENTUCKY REGION


KOHRS, Russell H., Department of Geology, Univ of Cincinnati, H.N. Fisk Laboratory of Sedimentology, 500 Geology Physics Building, Cincinnati, OH 45221, piperconan@hotmail.com

Well preserved Konservat Lagerst├Ątten can be found preserved by obrution deposits within the shales of the Cincinnatian Series. These deposits, formed as distal tempestites, can provide insight into the depositional processes which formed the meter-scale cycles present today. Obrution deposits provide a paleoecological snapshot of the composition of fossil communities prior to burial, due primarily to the rapid deposition of mud. This rapid mud deposition suggests that the shale portions of these cycles represent relatively short periods of time relative to the thinner, time-averaged, limestone cycle caps. Centimeter-scale characterization of the shale portion of the cycles within the Alexandria Submember of the Kope Formation was conducted at five sites following the inferred paleoslope toward the Sebree Trough to the northwest of the study area. Meter-scale cycles within this unit were correlated over 140 km while many of these single event beds were correlated over a distance of 10 to 40 kilometers. Two of these event Lagerst├Ątten occur within cycle 29 of the Alexandria Submember (cycle nomenclature of Holland et al., 1997), where whole Cryptolithus bellulus and Flexicalymene meeki have been found in two obrution deposits over a distance of 20 km. Other faunal epiboles occur within cycle 28 where Isotelus maximus and Triarthrus eatoni have been found to occur over similar distances. Other beds within the meter-scale cycles, such as thin grainstones and calcisiltites can also be correlated over these same distances. Some show changes in proximality as they are traced downslope as thin grainstones grading into calcisiltites and eventually into distal mud. The fauna which can be found within these deposits of the Kope Formation are restricted to a select few taxa. The excellent preservation of the fauna within obrution deposits and their common occurrence indicate that the shales of the Kope Formation were deposited rapidly, in a series of individual tempestite deposits in deep water well below storm wave base. Other beds within these cycles such as thin grainstones and hummocky cross-stratified calcisiltites suggest deposition within storm wave base. These deposits contrast sharply with the highly time-averaged limestones which cap these cycles which were formed over a comparatively long period of time near the upper level of storm wave base.