Freshwater Facies of the Lower Bakken Shale: Lost Observations Updated by New Palynology Redefine Lower to Middle Bakken Surfaces and Sequences

Abstract

Brackish to freshwater conchostracans (clam shrimp) in the upper part of lower Bakken shales in outcrops from Utah to Alberta have been known since the 1960s (Gutschick et al. 1962; 1970; 1978), and recently discussed by Myrow et al. (2015). Likewise their presence in exactly the same stratigraphic position in the Williston Basin was demonstrated by Thrasher (1987). Petroleum geologists and geochemists working in the subsurface have neglected these fauna and the significant implications they have for sequence stratigraphy.

Lower Bakken shale unconformably overlies the Three Forks Formation (and other formations) and can be subdivided into three units. The lower unit is thick marine shale with radiolaria overlain by a subtle surface that locally has a 1-2 cm thick fish bone and conodont-bearing phosphatic sandstone lag. This surface is overlain by thin black shale with articulated freshwater conchostracans, rare brittle stars, and Orbiculoidea sp., indicative of deposition in a restricted brackish-water environment. The conchostracan-bearing unit grades up into a very thin green shale with clams, crinoids, and diverse brachiopods. This green shale is in turn sharply transitional with overlying Middle Bakken silty wackestone.

Palynology studies from lower Bakken shales in western Montana show a change from marine-dominated species in the lower cherty marine shale to terrestrial-dominated species in the overlying conchostracan-bearing shale. These data confirm that the conchostracan-bearing shale was deposited in a brackish near-shore environment as indicated by the macrofossil assemblage. The lag sand separating these different environments is a subtle sequence boundary.

The basal marine unit should be considered its own sequence truncated by the overlying sequence boundary and conchostracan-bearing shale. This sequence boundary records a regional drop in base level that affected the entire continental shelf. The black brackish-water shale above the sequence boundary grades upwards into oxygenated, marine green shales and this package represents an LST or early TST (regional “passive transgression”). The contact with the overlying Middle Bakken silty limestone is a sharp flooding surface, not a sequence boundary as is commonly suggested. Instead it may represent a maximum flooding surface and HST turnaround. Up-dip transgressive Middle Bakken rocks can also rest on the lower marine shale unit such that the above surfaces are amalgamated.

AAPG Datapages/Search and Discovery Article #90323 ©2018 AAPG Annual Convention and Exhibition, Salt Lake City, Utah, May 20-23, 2018