Page 142 - Carbonate Facies in Geologic History

Alberta Basin Banks 129

Table IV-3. Correlation of early Late Devonian in Alberta
European
Rocky Mountains outcrops Alberta Basin subsurface
stage
Sassenach Alexo Famennian

Nisku
Mt.Hawk
a. a.
:::l :::l Ireton
0
0 "-
"- Southesk en gray-green
en Mt.Hawk ""C shale
c Leduc
QJ
QJ ..0 dolomite
E ""C Srr Duvernay Frasnian
- Cairn ) Perdrix 0 imestone shale
brown
0 0
.r. Maligne 3 <pcooking L.
.P latform 1st.
"- QJ
.- Flume -"
0
0 -I $woo ,% W,_,
~ limestone
U. :.c Hills shale-
~ limestone
QJ
In
exposed in these strata of the Cordilleran miogeosyncline, the mountain sections
offer valuable clues to the Alberta basin subsurface.
Three Devonian basins were discovered during petroleum exploration of the
prairie provinces of Canada. They lie between the Precambrian Canadian Shield
and the western Cordilleran geosyncline. From north to south, these are: the Hay
River, Alberta, and Williston. The basins were dotted with stromatoporoid build-
ups in Middle Devonian time, and their edges were lined by the facies. During a
regression toward the end of the Givetian, the basins were filled with evaporites.
Marine transgressions from the northwest occurred episodically but with ever-
increasing effect throughout the Frasnian until a major regression began in Fa-
mennian time. The Frasnian transgressions are represented in the Alberta Basin
by two major groups of formations: Beaverhill Lake and W oodbend. During each
of these depositional intervals, tectonic positive elements in the basement must
have formed the bases for extensive carbonate platforms, rimmed by massive
irregular stromatoporoid accumulations. Some of these appear as great halos
built out from major elements. Carbonate buildups also developed on multiple
linear and roughly parallel structural trends as well as on isolated equidimen-
sional ones, presumably along basement faults, and over horst blocks. Positive
gravity anomalies are known along some of these. The map on Fig. 11-19 indicates
the rather clear NE-SW tectonic grain displayed by the linear buildups and trends
of shelf margins. The buildups are chiefly flat lens-shaped bodies; their lower beds
grading into the surrounding shale. Marginal slopes are from 3/4 degree to 5
degrees and are generally (not always) steeper on the eastern or northeastern
sides, where more organically constructed boundstone is seen. Thickness of build-