Page 113 - Carbonate Facies in Geologic History

100 The Advent of Framebuilders in the Middle Paleozoic

described by Hoffman to the beginning of the Middle Ordovician, about
450 million years ago. At this time, in North America and around the Baltic
Shield, the first real "reefy" faunas began to develop. About 100 families are found
as fossils in Cambrian strata. Twice this many occur in the Middle Ordovician
despite the loss of two-thirds of the Cambrian trilobite families which constituted
the majority of the Cambrian life forms. Many representatives of these new fami-
lies appear first in small carbonate buildups. Bryozoans, rugose corals, stromato-
poroids, and tabulate corals constitute the important new Ordovician sessile
benthos capable of carbonate construction. Accessory organisms such as cephalo-
pods, trilobites, and brachiopods, and above all the crinoids and blastoids, be-
come significant on and around the buildups.
The change from Lower Ordovician to Middle Ordovician faunas occurred
during formation of a regional unconformity in North America. It was an event
almost of equal evolutionary consequence to the first appearance of shelly calcar-
eous tests in the Cambrian 150 million years earlier and the biological changes
ending the Paleozoic 200 million years later. The ability of these varied organisms
to form a definite ecological association, to accrete carbonate in their tissues at a
rapid rate, and to proliferate and diversify, was a significant event in earth history.
Large discrete masses of calcium carbonate were built up at continental mar-
gins and as smaller mounds around and within shallow shelves and basins. Circu-
lation became more restricted, encouraging evaporite deposition. At the same
time basin interiors within, and marginal to the craton, became starved of terrig-
enous sediment. With regional subsidence the basins became more sharply dif-
ferentiated from the shelves which, themselves, were built up by increased carbon-
ate accumulation. Up to this time, mainly physical processes of tidal flat construc-
tion had added carbonate sediments to these shelves. Now effective organic con-
struction became an important process. These early "reefy" faunas and floras offer
significant ecologic comparisons with the larger ones of the Mesozoic and the
present day. It is also of interest to observe the burst of evolution in this biota
during the Middle Paleozoic. It begins in the Middle Ordovician and declines
during Fammenian time in the Late Devonian.
The earliest carbonate buildups of the Middle Paleozoic have been well de-
scribed by Pitcher (1964) and Toomey and Finks (1969), who studied them in the
New England Chazyan beds along Lake Champlain, an area which extends
across the Canadian border north of Montreal in Quebec province. This shelf lay
west of the Appalachian geosyncline and north of the Adirondack-Frontenac axis.
It accumulated 300-1000 m of well-bedded carbonate, beginning in the earliest
part of the Middle Ordovician. Micritic mounds are distributed in these sedi-
ments, but observation of regional trends or evidence for localization of particular
mounds is not permitted by the scattered outcrops. It is evident that the mounds
formed on a shallow shelf in conditions of varying water agitation. All mounds
are small, from 2 to 8 m high at most. They are the same shape and size as the
alga-sponge mounds existing in the Lower Ordovician and Upper Cambrian, but
faunally they are much more complex.
Distinct biological assemblages are recognized in these mounds but a clear-
cut vertical succession of faunas is rare in the oldest Middle Ordovician. Figure
IV-4 is a schematic diagram of these mounds and faunas from Pitcher (1964).