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Sedimentary features of the crossfield member

1969; Canadian Society of Petroleum Geologists; Volume: 17; Issue: 4 Linguagem: Inglês

10.35767/gscpgbull.17.4.444

2368-0261

R. G. Metherell, L. E. Workman,

Geological formations and processes

ABSTRACT The Crossfield Member is a gas-producing zone in the middle of the Stettler Formation of Upper Devonian age. It averages about 100 ft in thickness and is recognized in a north - south belt, about 2to 12 mi wide, along the line of the Fifth Principle Meridian, Alberta, from Olds to south of Okotoks. The Crossfield consists of porous dolomite between the dense anhydrites and evaporitic dolomites of the Upper and Lower Stettler members. It forms a shallow shelf deposit lying between open marine sediments of the undifferentiated Stettler to the west and evaporitic intertidal and supratidal deposits to the east. It is divisable into three zones, each with somewhat different facies types, from the bottom upward as follows: 1. Basal shallow mud and lenticular algal accumulation; 2. A dominant Stromatoporoid and gastropod bank; and 3. A dominant algal bank. From the dolomitized open marine calcilutites and pelletoidal sediments to the west, the lowest zone grades eastward into dolomitized calcilutites with irregularly distributed interbeds and lenses of algal muds having fenestral porosity. These give way eastward to a dense muddy shoal facies on the border of the supratidal shelf. The middle zone grades eastward from the open marine sediments through crinoidal-stromatoporoidal-brachiopodal grainy fore-shelf and mud facies to a muddy shoal and finally evaporitic dolomite and anhydrite sabkha deposits. East of the typical open marine deposits, the upper zone has a grainy mud facies, similar to the grainy fore-bank facies of the middle zone. In certain areas; this facies is flanked on the east by dolomitized calcarenite or originally fine to medium limestone grains, grading eastward through light gray dolomitized algal mud with fenestral porosity to muddy shoal facies. Commonly the calcarenite and algal mud is absent and the grainy mud facies grades directly eastward into the muddy shoal facies. East of the muddy shoal facies in each case are the evaporite sabkha deposits. It appears that the lower zone is more or less variable in deposition, the middle zone is transgressive eastward, and the upper zone regressive westward.