Abstract

The Upper Jurassic (Callovian-Oxfordian) Roseray Formation was deposited on the northwestern margin of the Williston Basin. During this time the Williston Basin was connected to the open ocean and was subject to eustatic fluctuations of sea level. The sea level curves indicate a drop at 155.5 m.y.a. and predict the presence of a sequence boundary during Roseray deposition. These eustatic changes may also be coupled with tectonic effects (e.g., variation in subsidence rates). Fluctuations of relative sea level in the Roseray Formation are recorded by depositional breaks marked by abrupt juxtapositions of facies, pebble lags, eroded mudstone rip-up clasts and mineralization in a relatively conformable succession of sedimentary rocks.

The sequence boundary which formed during a relative sea level fall defines the base of the Roseray Formation and abruptly terminates open marine deposition in the underlying Rush Lake Formation. The depositional package which comprises the Roseray is divided into two successions on the basis of the parasequence stacking patterns. The lower part of the Roseray is characterized by a progradational stacking pattern suggesting deposition in an overall base level fall. The upper part of the Roseray is characterized by a retrogradational stacking pattern suggesting deposition during a base level rise. The Masefield Shale unconformably overlies the Roseray Formation and marks the end of Roseray sandstone deposition in this area.