Page 121 - Carbonate Facies in Geologic History

108 The Advent of Framebuilders in the Middle Paleozoic

JEFfeRSONVILLE ENNETH / KOKOMO

Fig. IV -8. Simple Middle Silurian mound near Georgetown, Indiana in clastics-free belt of
Lowenstam. Original relief low, composed merely of micrite with bryozoans and crinoids.
Lower diagram not to scale. Mound is about 2 km across and 40 m thick. Core holes outline
mound through glacial drift and partly under thin cover of Devonian Jeffersonville and
Kenneth-Kokomo Formations. Petrography by O.P.Majewske

out that reef construction is later in time on the Indiana shelf than previously
realized. It is equivalent in part to the Salina evaporites of the Michigan basin.
Shelf buildups are varied, types being controlled by location on the shelf which
sloped south into the Illinois basin (Fig. IV-7). Five types are described below.

1. Algal spongiostrome stromatolitic mounds in Ohio and Wisconsin and northern Indiana
(Textoris, 1966; Soderman and Carozzi, 1963; Textoris and Carozzi, 1966)
These mounds, which are small (some less than 3 m high and a few tens of meters across),
developed in a restricted marine environment far up on the shelf on the perimeter of the
Michigan basin. The algal stromatolite beds are brecciated in places, owing to intermittent
desiccation and wave action. At some localities, the stromatolite caps a mud mound of the
type described below.

2. Bryozoan-mud mounds with abundant stromatactoid structures
These correspond to "reefs" in the semirough water stage of Lowenstam (1950, 1957) and
to stage 2, microfacies 3, of Textoris and Carozzi (1964). They formed below active wave base
and grew to heights of some tens of meters, a figure partly determined by water depth (wave
base).