Migrating Depozones and Diachronous Hot Shales: Upper Cretaceous (Santonian) Puskwaskau Formation, North-Central Alberta

Abstract

There are relatively few studies that have attempted to understand the organization and regional distribution of mud-rich strata in foreland basin settings. The Upper Cretaceous (Santonian) Puskwaskau Formation spans a large region of the Western Canada Foreland Basin and provides an opportunity to study the lateral variability of a mudstone-dominated succession. Regionally-extensive high-resolution subsurface correlations reveal the stratal stacking pattern of the Puskwaskau Formation in a distal foreland basin setting (hundreds of kilometers offshore). Rocks of the Puskwaskau Formation are organized into 14 regionally-mappable allomembers, labeled A-N in ascending order. These 14 allomembers are grouped into three larger stratigraphic ‘units’, with each unit interpreted to have been deposited in a distinct flexural depocenter. New stratigraphic correlations show that the Puskwaskau Formation undergoes marked thickness changes over distances of 100 km parallel to the orogen and 500 km perpendicular to the orogen. Units 1, 2 and 3 have wedge-shaped geometries. Unit 1 thins markedly towards the north (parallel to orogen) and towards the east (perpendicular to orogen), in the direction of the forebulge. In the northeastern part of the study area, allomembers of Unit 1 thin significantly over distances of only one to two townships. This thinning trend is interpreted to have been controlled by the episodic vertical movement of fault-bounded basement blocks. The lower allomembers of Unit 2 thicken in the direction of the forebulge, whereas the upper allomembers of Unit 2 thin in the direction of the forebulge. The lower allomembers of Unit 3 thin in the direction of the forebulge, whereas the upper allomember (‘L’) of Unit 3 thickens in the direction of the forebulge. These thickening and thinning trends suggest that accommodation was developed locally within the basin. Active thrusting in the orogen may have generated local regions of subsidence, resulting in the lateral shifting of depocentres. The thickening of allomember ‘L’ in the direction of the forebulge suggests possible orogenic unloading (reduced thrusting), and in turn, reduced subsidence in the foredeep. In the distal part of the study area, two, diachronous “hot shale” zones become prominent. These two “hot shale” zones may be interpreted to represent geographically localized intervals of minimal clastic accumulation, related to the trapping of clastic sediment in the proximal foredeep.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016