Defining the nature and location of a Late Devonian–Early Mississippian pycnocline in eastern Kentucky

Abstract

The pycnocline, the intermediate layer in a tripartite, density-stratified water column, is an important element in recent explanations of the origin of anaerobic sediments. Dysaerobic sediments deposited where the pycnocline intersects the bottom are distinct from overlying aerobic and underlying anaerobic sediments and were used to define the position of the paleopycnocline on a Late Devonian-Early Mississippian sea bottom in eastern Kentucky. Surface and subsurface mapping of the Bedford-Berea sequence and equivalent parts of the New Albany and Chattanooga Shales reveals approximately coeval aerobic, dysaerobic, and anaerobic facies that permitted mapping of the paleopycnocline, apparently the first time the actual sea-bottom position of a paleopycnocline has been defined. The Bedford-Berea (Devonian-Mississippian) sequence is situated between the black Ohio Shale (Devonian) below and the black Sunbury Shale (Mississippian) above. The black shales reflect deposition in a deepening, transgressive sea where anaerobic conditions were easily established and maintained. The intervening Bedford-Berea sequence, however, represents deposition in aerobic and dysaerobic environments during a shallowing, regressive episode as deltas prograded to the south and west in eastern Kentucky. The gray sandstones, siltstones, and shales of the Bedford-Berea sequence have been interpreted as pinching out between the black shales above and below in east-central Kentucky. Each of the above units, however, has a distinctive signature on gamma-ray logs. Comparison of gamma-ray logs and radioactivity profiles with corresponding cores, drillers9 logs, and exposures indicates that black-shale equivalents are present beyond the point where the gray Bedford Shale pinches out; this also is supported by limited biostratigraphic data. This transition from gray Bedford shales to black-shale equivalents marks the approximate position of the ancient pycnocline and has been mapped in the subsurface and on the outcrop in eastern Kentucky. The ancient pycnocline closely parallels the Bedford-Berea delta front, suggesting major control by the delta. The continued influx of oxygen-rich, sediment-laden waters from the delta apparently prevented the northward and eastward migration of the pycnocline up the delta front. The introduced oxygen and sediment, however, were effective only in breaking up the pycnocline over a distance of ∼25 to 40 km from the active delta fronts. Stratigraphic, paleontologic, and geographic constraints indicate that the paleopycnocline was not horizontal but sloped northeastward from shallower waters near the Cincinnati Arch to deeper waters off the delta fronts.