EVENT DEPOSITION IN SHALES – WOODFORD SHALE, OKLAHOMA, USA, AND BJøRKåSHOLMEN FORMATION, SWEDEN

Shales from two Paleozoic marine settings, the Devonian Woodford Shale of Oklahoma, USA, and siliciclastic mudstone intercalations into the Ordovician Bjørkåsholmen Formation carbonates of Sweden, exhibit distinct millimeter- to sub-millimeter laminae (present thickness) in thin section whereas in outcrop they are massive. The Woodford shows a two-fold subdivision of laminae with a dark-colored lower and a light-colored upper part. The lower portion of the lamina contains abundant organic matter, pyrite and, locally, silt-size detrital quartz and carbonate grains at its base. In the central to upper part of these laminae, pyrite is less common than at the base. Planolites burrows cluster at the lamina top. Phycosiphon isp. fecal strings and Tasmanites algae are present throughout, but the fecal strings are less frequent in the pyrite-rich lower part of laminae.

The siliciclastic mudstones in the Bjørkåsholmen Formation either contain abundant shell debris and phosphate clasts, or consist of intercalated light- and dark-colored bands of irregular geometries that are one to several millimeters thick. Phycosiphon isp. is present throughout.

In the Woodford Shale, the laminated texture and the presence of Planolites burrows just at the lamina tops suggests lamina deposition in pulses, followed by non-deposition and limited bioturbation. The siliciclastic mud was carried as bed load indicated by local, basal silt-size lags, or as fluid mud. In either case, deposition was a result of an event, probably storms, from which was deposited one discrete millimeter-thick mudstone lamina. This mudstone lamina was subsequently colonized from the top downward in a dysoxic water column resulting in Planolites traces only at the top, and fecal strings diminishing in abundance downward.

The Bjørkåsholmen Formation mudstones also show lag deposits, both shell debris and phosphate clasts, which probably originated from currents that moved by bed-load transport. Similar to observations from the Recent Eel delta (USA), the Bjørkåsholmen Formation mudstones were most likely deposited by a series of storm events. This study therefore demonstrates the significance of event deposition (not suspension settling) in the accumulation of important source rocks and unconventional reservoirs, and under dysoxic-oxic conditions.