Page 378 - Carbonate Facies in Geologic History

Origin and Orientation of Mounds 365

northern end of Isla Blanca Bay, northeast Yucatan, and Bulkhead shoal at the
southern end of Chetumal Bay, British Honduras. These are caused by dampen-
ing-out of tidal currents and swells from the open sea and the mechanical accu-
mulation offine lime-mud sediment in slack water.
More sharply defined, bread-loaf, elongate mounds are possibly shaped by
longshore currents. These chains of mounds parallel the coast lines and may form
in multiple rows. Analogs from the Holocene include Rodriguez and Tavernier
Keys off the Florida Keys (Turmel and Swanson, 1972). These same features may
occur in groups across the shelf areas, many with axes parallel to the depositional
strike and hence to the ancient coast lines. Detailed studies of some areas show
that orientations of ellipsoidal mounds may also be normal to the shoreline. Such
trends may be tidal deltas at the mouths of passes through the Pleistocene rock
barriers or reefs which partically enclose the shelf lagoons. Tidal bars which may
occur within wider passes are more stream-lined than the delta accumulations.
These can vary in composition from clean sand to accumulations of lime mud and
bioclastic debris stabilized by veneers of organisms. Fine examples of Holocene
tidal deltas and tidal bars in passes may be observed between some of the middle
and lower Florida Keys. The 10 km wide fringe of lime mud banks lining the
mainline shore of Shark Bay, Western Australia display transverse tidal channels
oriented normal to the coast (Davies in Logan et aI., 1970). Accumulation results
from interaction of tides and longshore drift with organic carbonate production
on and trapping by mats of marine grass. Such transverse trends of micritic
buildups are not common in the geologic record but are known in the Pennsyl-
vanian strata of the Paradox basin and midcontinent.
Even mounds developed below wave base and out of reach of the surface
currents may have well-defined trends. The mounds described by Neuman et al.
(1972) off Bimini in the Florida channel are apparently current-formed bodies.
Currents are observed here at depths of 700 m; the elongate shape of the bodies is
distinctive. In contrast, the Lower Carboniferous Waulsortian mounds, devel-
oped downslope of the preexisting scarps, are more or less circular. In addition,
they often occur across the basins in lines whose trends may be structurally
controlled. Many deeper water mounds, as well as those located in shallow basins,
lack apparent orientation. Swarms of pinnacle reefs appear in complex patches on
platforms in some basins (Klovan, 1974). Purdy (1974a) has proposed that such a
pimply or labyrinthine pattern is caused by growth on submerged karst surfaces.
He notes such patterns within the Holocene-Pleistocene large Alacran atoll on the
Campeche bank and on the wide shelf behind the British Honduras barrier reef.
Numerous examples from the geologic record show that such patches of pinnacle
reefs cluster on platforms or terraces which may be marginal to basins and later
drowned by submergence. These include the West Irian Miocene (New Guinea);
the north central Texas Canyon buildups; the Cooking Lake platform with Late
Devonian Leduc reefs of Alberta, Canada; the Devonian Presqu'ile reefs of north-
ern Alberta, and the Michigan basin Silurian.
At present very little data exist on the origin and orientation of basinal build-
ups. The above discussion of mound genesis and orientation emphasized their
origin as hydrologic accumulations, drawing heavily on recent sediment analo-
gies. In geological occurrences it is always difficult to explain the localization of