Page 145 - Carbonate Facies in Geologic History

132 The Advent of Framebuilders in the Middle Paleozoic

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sec 24 sec 15 67N.l0W5 sec 32 sec 29
66N.l0W5
Fig. IV-22. Upper Devonian carbonate bank, Swan Hills oil field, Alberta. After Fischbuch
(1968, Fig.11 a). Bank is a buildup from normal marine limestone to a shoal of Amphipora ~
and dendroid stromatoporoid (branching symbol). This was covered by layers of massive
encrusting stromatoporoids I§I and capped by grainstone as the bank accumulated into wave
base. The Roman numerals indicate stages of growth in time-stratigraphic units. These are
defined by thin shaly units traceable across the buildup. The upward facies progression is
repeated laterally from bank edge to bank interior in most of the time-stratigraphic units. For
legend see Fig. III-1. Illustration courtesy of author and Canadian Society of Petroleum
Geologists
bank of W oodbend age lying east of one of the linear trends which crosses the
Alberta basin (Rimbey-Led uk-Meadowbrook). Figs. IV -20 and IV -21 indicate the
profile and paramount sedimentary facies of this bank. Figure IV-22 is based on
Fischbuch's reconstruction of an older and flatter Swan Hills bank (1968) but
plotted with a reduced vertical exaggeration.
Despite the differences in age, size, and shape, the Late Devonian Alberta
banks are all generally of low profile, are flattish, isolated buildups, with essen-
tially level tops containing deposits of very shallow lagoons. In places they devel-
oped on wider platforms, sticking up as flattened "pinnacle reefs." Marine circula-
tion was restricted and winds, waves, and currents very moderate. Oolite is essen-
tially lacking, indicating quiet water with not much tidal action. Multiple rows of
buildups also indicate reduced effects of hydrographic controls. Evaporite deposi-
tion occurred only on the interior of the broadest shelves to the south. Tidal flats
are indicated in several places by the fenestral laminite facies. The sediments also
share the same upward changing sequence of organisms described by Lecompte
for the smaller bioherms of the Dinant basin, Belgium.

Deposits of Steep Bank Edges Found in the Outcrops of the Canadian Rockies

Several major carbonate banks have been studied in outcrops of Late Devonian
strata in the Canadian Rockies. These range from banks with very little relief
(Dooge, 1966; Cook, 1972) and gradational facies over considerable distance, to
much steeper shelf margins with more than 100 m relief and 2-10 degree slopes