THE SIGNIFICANCE OF EROSIONAL FEATURES FOUND IN THE MIDDLE DEVONIAN GENESEO SHALE

The Middle Devonian (Givetian) Geneseo Formation superficially appears to be a monotonous organic-rich shale. It has previously been understood to reflect a major transgression associated with enhanced organic matter preservation due to deeper water stagnant-anoxic conditions. This unit does, however, exhibit an abundance of thin silty laminae that can be interpreted as current produced on the basis of recent flume studies. Moreover, discovery of additional features indicative of intermittent erosion and reworking, provide a new perspective for the understanding of basin dynamics, sedimentation, and water depth associated with Geneseo deposition.

Petrographic and small-scale sedimentary features were examined in a core and in thin sections for the entire Geneseo succession, in order to arrive at an improved understanding of mudstone facies and paleodepositional environment. The lower Geneseo is a banded black shale, representing more rapid deposition and surficial mixing by benthos (“cryptobioturbation”). The middle Geneseo exhibits multiple dark gray-black shale cycles, bounded by erosional surfaces often marked by a thin lag. These, interpreted as parasequences, probably reflect fluctuations in sea-level. Dark gray muds display thin silt lags and laminae at the base, and an increase in macroscopic bioturbation and homogenization upwards, grading into the pyritic black muds that exhibit lamination, erosional silt lags and “cryptobioturbation”. These cycles decrease in thickness upsection and grade into dark gray shales with scours, wave and current ripples, and macroscopically visible bioturbation. The upper Geneseo is marked by the Lodi Limestone, a thin silty carbonate concretionary unit that represents sediment starved conditions.

The Geneseo shale succession reflects the interplay between sediment supply and accommodation during a forced transgression, sequestering sediments further onshore and giving rise to an increase in offshore bottom current reworking and erosion. Erosional features and small scale sedimentary structures in the base of parasequences suggest an increase in sediment exposure and reworking (amalgamation) due to sediment starvation, and also is consistent with lower preservation potential of bioturbation features in basal portions of parasequences.