ABSTRACT: Sequence Stratigraphy of the Upper Cretaceous Milk River and Lea Park Formations, Alberta

O'CONNELL, SHAUN C., Alberta Geological Survey, Edmonton, Alberta, Canada, JANOK P. BHATTACHARYA, Atlantic Richfield Co., Plano, TX, and DENNIS R. BRAMAN, Royal Tyrrell Museum, Drumheller, Alberta, Canada

This study examines the sequence stratigraphic relationships between the Alderson member and the Milk River Formation, incorporating data from approximately 450 well logs and 500 m of core from 9 wells. The Upper Cretaceous Milk River Formation in southern Alberta and Saskatchewan forms a northward tapering sandy clastic wedge. It consists of nonmarine and shoreline deposits that pass northward into marine shales, siltstones, and fine-grained sandstones belonging to the Alderson member of the Lea Park Formation. The Milk River is unconformably overlain by marine shales of the Pakowki Formation. The Alderson Member has been subdivided into ten lithofacies varying from coccolith-rich nonbioturbated shales deposited in oxygen-depleted conditions, to fine-grained, moderately well sorted s ndstones containing wave ripples and a diverse and abundant ichnofossil assemblage. These lithofacies are arranged into coarsening upward parasequences, of which several are capped by chert pebble horizons. Bentonite layers provide important time-markers for correlation purposes.

Parasequences in the Milk River Formation, interpreted as a highstand systems tract, downlap to the north onto the 1st White Speckled Shale, which is interpreted as a condensed section. The Milk River/Pakowki unconformity is marked in outcrop by a chert pebble layer. Palynological data suggest a significant gap of between one and two million years. This unconformity is interpreted as a sequence boundary and dips seaward underneath the Alderson member, which is interpreted as a lowstand systems tract. Parasequences within the Alderson member onlap this unconformity in a southward direction. Palynological data suggest a late Campanian Pakowki age for the Alderson member and support the log-based correlations presented here. Transgressive erosion at the top of the Alderson lowstand may h ve enhanced the unconformity farther to the south.

AAPG Search and Discovery Article #91012©1992 AAPG Annual Meeting, Calgary, Alberta, Canada, June 22-25, 1992 (2009)