Page 132 - Carbonate Facies in Geologic History

General Summary of Later Devonian Facies 119

stromatoporoid evolution occurred over the whole world, establishing the culmi-
nation of Middle Paleozoic reef biota. Colonial rugose tetracorals were added as
reef forms to the dominating tabulates. This assemblage took many forms and
existed in many tectonic situations. Almost all types of carbonate buildups were
represented from stromatoporoid-coral capped micrite mounds in basins and
geosynclinal troughs to micrite mounds, faro-like patch reefs on shelves, and
gigantic sprawling banks across marginal cratonic basins. Linear barrier reef
trends were also represented in Devonian time.
Carbonate buildups of latest Silurian and earliest Devonian age are very rare,
despite extensive Middle Paleozoic exposure in the northern hemisphere. Howev-
er, reefs whose age is bracketed between Ludlovian and Emsian Series are known
in central Asia. Stauffer (1968) described a narrow Siluro-Devonian reef trend
25 km long near Nowshera in northeastern Pakistan, close to the Indus River
near Kashmir and Afganistan. This is probably part of an east-west trending
platform edge although regional relationships are not clear. It consists of individ-
ual elongate mound-like cores tens of meters thick and hundreds of meters long.
Flanking facies consist of crinoidal beds and reef-derived breccia, both lithoclastic
and bioclastic. The latter consist of crinoids and pieces of dendroid favositid
corals. An interesting faunal sequence is described from top to base of the reef
cores, indicative of growth into wave base by the communities:
Top layer4. Tabular massive stromatoporoids more than 30%, abundant orthoconic
nautiloids. No algae. Other coelenterates not common. Common gastropods.
Layer 3: Tabular massive stromatoporoids about 30%, rugose corals, massive Favosites,
crinoids and gastropods common. No algae.
Layer 2: Abundant 1hamnopora colonies and delicate dendroid favositids. Massive tabu-
lar stromatoporoids 25%, rugose corals and crinoids common.
Layer 1: 1hamnopora colonies and dendroid favositids, less than 20% massive tabular
stromatoporoids. Common rugose corals. Abundant brachiopods.
This sequence rests on a sparsely fossiliferous micritic limestone with some
brachiopods, crinoids, and scattered stromatoporoids.

General Summary of Later Devonian Facies
Pioneer ecological studies of Late Devonian buildups derive from paleontologic
and stratigraphic work by Marius Lecompte in the Dinant Basin, Belgium. Vast
petroleum reservoirs were discovered in dolomitized and partly altered limestone
strata in the Late Devonian of the subsurface of western Canada about 1950. This
spurred the expansion to North America of the detailed biological and petro-
graphical knowledge amassed by Lecompte. The discovery and description of a
beautifully exposed reef trend of the same Late Devonian age in western Australia
later added immensely to our knowledge of such strata. The Moscow basin also
contains Devonian petroleum reservoirs. No other reefy strata have been so
thoroughly analyzed in so many different parts of the world as the Givetian-
Frasnian beds have been from 1950-1970.
In all the buildups discussed below, the biological assemblage is basically
similar. A generalized review of microfacies and lithofacies is given which com-
bines petrography, organic content, and sedimentary structures. World-wide ex-