REGIONAL-TEMPORAL CHARACTERISTICS OF PALEOZOIC, TRANSGRESSIVE, BASAL BLACK-SHALE DEPOSITS: MODELING DIACHRONOUS ONLAP PROCESSES ON OXYGEN-DEFICIENT, EROSIONAL-CORROSIONAL SUBSTRATES ON LAURENTIA

Many Paleozoic black-shale units on Laurentia are particularly noted, not only for their great regional/spatial extent, but also for their dramatically sharp, basal contacts, which display both temporal and lithologic discordance with underlying units. These contacts are characterized by distinctive, dissolutional placers of quartz sand, phosphatic debris, and detrital pyrite, which lack associated carbonate allochems, reflecting extended exposure on a corrosional, oxygen-deficient seafloor. Where limestones floor black-shale units, significant volumes of carbonate may have been removed, locally resulting in dramatic corrosion-related substrate relief.

The Upper Ordovician Indian Castle Formation (upper Utica Shale) distinctly onlapped westward, up an east-sloping, variably faulted, sediment-starved erosional-corrosional ramp during tectonic enlargement of the Taconian foreland basin, as indicated by westward descent of tephras and zonal graptolites onto this contact. Similarly, tectonic expansion of the Devonian orogenic basin during a major eustatic transgression, triggered the well documented diachronous onlap of Late Givetian laminated black-mud deposits of the Geneseo Formation. Corrosion and reworking of allochems upslope from the Geneseo mud onlap limit led to the concentration of diachronous placers of detrital pyrite (Leicester Member) in erosional furrows that were later covered over by the advancing mud-edge during transgression.

New work on the Late Famennian Cleveland Member in Ohio, shows that the Skinner’s Run pyrite bed, flooring this black shale unit, is a younger analog to the Leicester Member. New correlation work shows that the Cleveland Member, temporally linked to the global Dasberg transgression event, onlaps a sloped erosion surface, southeastward toward an inferred basin margin across the Cleveland metropolitan area, such that different internal Cleveland Member divisions can be seen to roof the pyritic lag in different places. These observations suggest that other widespread black shale-roofed contacts in cratonic settings can be shown to be measurably diachronous even on a local scale.