Upper Devonian Formations Across the Peace River Arch, West-Central Alberta [Abstract]

ABSTRACT Published geological information indicates that the Devonian rock succession onlaps a pre-Devonian paleotopographic high region located in west-central Alberta, named the Peace River Arch. The more elevated parts of this arch were emergent during the Devonian and acted as a source for detritus. A succession of coarse grained continental and nearshore, arkosic sandstones overlies and surrounds the arch. An interbedded succession of Upper Devonian carbonates and shales onlaps the sandy facies, and fringes and overlies the arch. Many of the Devonian units surrounding the arch thin to zero or change facies over the arch, so that it is difficult to sketch a depositional history based on a sedimentary and stratigraphic analysis of the onlapping beds. The depositional onlap pattern of the oldest beds indicates that the carbonates and shales of the Beaverhill Lake Group are in lateral facies contact with the nearshore, sandy Granite Wash facies that fringes the arch. The Woodbend Group overlies the Beaverhill Lake Group and includes an older reefal mound facies (the Leduc Formation), a dark inter-mound shale facies (including the Majeau Lake and Duvernay formations) and a younger, greenish grey shale facies that overlies the dark shale facies and abutts and overlies the carbonate mounds (the Ireton and Fort Simpson formations). Available information suggests that the Winterburn equivalents that overlie the Leduc fringing reef facies also overlie most of the area underlain by the arch, and onlap the Granite Wash facies in the higher parts of the arch. The peloidal and argillaceous limestones of the Wabamun Group overlie the carbonates of the Winterburn Group. Several map units of the Wabamun can be traced across the arch as being in facies contact with the Granite Wash sandstone. The Wabamun is locally absent in the crestal region of the Peace River Arch. The uppermost Devonian rocks in west-central Alberta belong to the Exshaw Formation. They comprise a relatively thin unit of dark grey, radioactive shale that includes a bentonite layer. It can be demonstrated that the Exshaw and the uppermost unit of the Wabamun Group are locally absent over the arch, presumably because of non-deposition. By removing the regional dip to the southwest and the compactional curvature across Peace River Arch, one can calculate second order residuals and draw a contour map based on the plotted values. This contour map outlines the fault-controlled depressions that are locally present in the axial region of the Peace River Arch. The depressions are filled with rocks of Mississippian age and attest to a later period of crustal extension. End_of_Record - Last_Page 173-------