Record of Sea Level Changes as Reflected in Facies Distribution Within Cardium Formation (Upper Cretaceous) of West-Central Alberta

James M. Rine, Grant Bartlett, Kenneth P. Helmold

A relative rise in sea level during late Turonian to early Coniacian time was a major factor controlling distribution of lithofacies within the Cardium Formation. Sea level variations controlled deposition of the two main subsurface units of the Cardium, the regressive Cardium sand and the overlying transgressive Cardium zone, and also influenced distribution of lenticular sandstone to conglomerate bodies that make up the majority of reservoirs within the Cardium Formation. The Cardium sand unit is a sand sheet that was deposited primarily in a strand-plain setting. It averages 9 m thick and extends for 80 km from the foothills of the Rockies eastward to where it thickens (up to 26 m) and abruptly pinches out into marine shales. East of this pinchout are lenticular bodies of inner-shelf sandstone and conglomerate. Overlying the Cardium sand is the Cardium zone, which consists, through most of the study area, of marine shales but also contains some continental and marginal marine sandstones in the northwest corner of the area.

The proposed scenario of how relative sea level changes affected Cardium facies distribution is as follows: (1) the Cardium sand was deposited as a prograding strand plain during a gradual sea level fall or stillstand, (2) progradation of this sandstone sheet ceased and its seaward edge was reworked landward as a result of a slight rise in sea level (5 to 10m), (3) reworking of the eastern edge of the sandstone sheet furnished sand and pebbles that were transported seaward and formed into shelf-sandstone bodies, (4) the Cardium sand interval was inundated and deposition of Cardium zone sediments began with an increase in the rate of sea level rise.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.