Page 212 - Carbonate Facies in Geologic History

Orientation of Mounds Relative to Shelf Margins and Their Origin 199

5. When the top of a mound remained at, or close to, wave base for a consid-
erable time, extensive flank beds formed owing to abundant production of bio-
clasts from organisms on the mound top. These commonly include small mobile
foraminifera, fusulinids, tubular encrusting foraminifera, Komia, etc. Volumetri-
cally, these steeply dipping foreset beds may constitute more than half the bulk of
the massive "reef' limestone.
6. In some mounds on steep slopes, a marine talus breccia or conglomerate is
present down the seaward flank of the mounds and contains lithoclasts and
bioclasts derived locally from the mound.
7. Commonly, after construction of a ramp or platform by the combination of
core and flanking beds, any period of stabilized sea level resulted in the produc-
tion of a horizontal capping bed of shoal grainstone, often cross-bedded and ooli-
tic. This bed commonly overrides the top of a mound-flank bed complex.
Figure VI-25 illustrates an idealized mound with the seven component facies.
Such a sequence of related facies is seen in many other mounds in other parts of
the geologic column and are summarized in Chapter XII.

Orientation of Mounds Relative to Shelf Margins and Their Origin

Orientation and mound origin are interrelated and are important in both explo-
ration and oil field development. Many individual mounds in the geologic record
(e.g., around the Oro Grande basin, New Mexico) are oriented with long axes
parallel to the trend of depositional strike and shelf margins. Also, there are some
at right angles to the general facies trend, e.g., Ismay field in the Paradox basin of
Utah, and perhaps in the outcrops of some mounds in the Big Hatchet Mountains
in New Mexico on the eastern flank of the Pedregosa basin. The elongate mound
shapes, their consistent orientation in anyone area, and the detrital sediments in
their basal beds would indicate an origin as bars or sedimentary piles formed by
currents or wave action and later colonized by the platy algae and/or tubular
foraminifera. Tidal bars in passes, terrigenous bars in off-delta areas, piles of
molluscan or crinoidal debris all could have afforded "starter areas" for organi-
cally induced lime mud accumulations.
Analogy with the modern Florida Reef Tract accumulations is significant
here. Tidal pass bars between the middle and lower Florida Keys and in the
"Safety Valve" of Biscayne Bay, are oriented normal to the coast line, whereas
Tavernier and Rodriguez bioclastic mud banks on the seaward side of the Keys
have an orientation at a low angle to that of the coast. These two banks may have
been formed at slight slope breaks and shaped by longshore currents. Tavernier
and Rodriguez Banks make a good analogy with many Late Paleozoic mounds.
They were formed as bioclastic lime mud accumulations in 3-5 m of water, fol-
lowing the post-Wisconsin marine transgression, grew to wave base, and are
capped by a bioclastic debris formed from sessile organisms growing abundantly
on the windward (seaward) side. These include corals, green algae, and a red alga,
Goniolithon, with a growth form much like that of Komia (Baars, 1963). Their
present shape and dimensions, except for thickness, are about those of many platy