AAPG ANNUAL CONFERENCE AND EXHIBITION
Making the Next Giant Leap in Geosciences
April 10-13, 2011, Houston, Texas, USA
Offshore Transport of Mud by Combined Flows: Upper Cretaceous, Western Interior Seaway, Northern Alberta, Canada
(1) Geology, Memorial University of Newfounland, St John's, NF, Canada.
(2) Department of Earth Sciences, University of Western Ontario, London, ON, Canada.
The Kaskapau Formation (U Cenomanian-L Turonian) is a mudstone-dominated clastic wedge deposited in NW Alberta during a phase of relatively rapid basin subsidence and sedimentation. Abundant exposure and subsurface wireline log data permit reconstruction of facies belts and subsidence patterns across a broad, very low-gradient ramp extending >300 km from foredeep to forebulge. Twenty-eight high-frequency sequences (allomembers) were mapped across the basin. Allomembers 5-17 were examined in outcrop along the Peace River, and in a core. Allomembers 7-18 are clay- and organic carbon-rich and encompass the ‘Second White Speckled Shale’. This package records drowning of the forebulge by water probably not more than10-30 m deep. The contemporaneous shoreline lay 200 -250 km to the SW.
All the studied allomembers are muddy and coarsen upward and can be traced across the foredeep into wave-dominated sandy nearshore sequences in the SW. The top surfaces of the distal sequences are commonly marked by a few, cm-scale wave-rippled beds of very fine sandstone, mm-scale shell pavements or phosphatic lags filling gutter casts. The lower parts of the parasequences, in allomembers 5-18, are composed of silt-bearing clay-rich mudstone to clay-bearing silt-rich mudstone. In some units, lamination is only poorly developed, and the units have been homogenized. At other levels, the sediment shows fine sub-mm to mm-scale parallel laminae variously composed of silt-sized quartz and clay. Silt laminae may also form wave and combined-flow ripples (1-2 mm thick) that are present at the base of normally graded beds. Much of the clay in these units is organized into floccule aggregates that have probably been reworked from the underlying muddy sea-floor.
Allomembers 7-18 were probably deposited > 200 km from the nearest shoreline; hence it is necessary to infer significant advective dispersal of mud across this shelf. Turbulent, storm-driven combined flows are likely to have been the dominant process responsible for advective transport of clastic detritus here. Texturally, as much of this material is organized into coarse silt-sized aggregate floccules composed predominantly of clay minerals, and organic matter, it is likely that much of this material was transported in bedload. The presence of these clay aggregates, coupled with ripple and parallel lamination, indicates that this mud-dominated shelf experienced intermittent high energy conditions.