Page 390 - Carbonate Facies in Geologic History

Review of Tectonic Settings for Carbonate Buildups and Cycles 377

There should be modern analogs to the categories defined by the above classi-
fication because the same descriptive parameters of megatectonic setting, trend
and shape of constituent sedimentary bodies are applicable to Holocene carbon-
ates. The problems of recognizing such analogs are greater than might be sup-
posed, however, for several reasons: (1) Our knowledge of Holocene carbonate
sedimentation is apt to be merely two-dimensional; in only a few places do we
have sufficient borehole data to enable us to determine thickness and facies and
Holocene-Pleistocene geological history. Geographic setting is more easily stud-
ied than tectonic framework. Of the half-dozen Holocene carbonate areas
known today, only in the Florida and British Honduras regions is a partial three
dimensional picture available. This prohibits the determination of rate of subsi-
dence and an evaluation of recent tectonic activity. (2) Because of Tertiary world-
wide tectonic activity and the relatively high stand of continents at the present
day, only a few small, shallow, epicontinental seas exist. These are floored with
only a thin veneer of sediment deposited within the last 5000 years or with no
sediment at all. (3) Indeed, no place has been discovered on earth where normal
neritic carbonate sedimentation has proceeded uninterrupted from Pleistocene to
Holocene. This is because the drastic Wisconsin-Wiirm glacial lowering of the sea
level (more than 100m only 15000-18000 years ago) laid dry all of the stable
shelves so characteristic of the carbonate producing regime. Additionally, conti-
nental shelves drowned by the recent rise of sea level have received hardly any
sediment since the stabilization of sea level at its present level 5000-7000 years
ago. Carbonate buildups at the margins of most of the shelves have not had
sufficient time in which to reestablish.
In summary, there are hardly any continental seas or intracratonic basins to
serve as modern tectonic models and modern shelves are strongly influenced by
the recent sea-level change and by purely oceanic hydrology and climate. With
recognition of these complications and limitations, the following attempt is made
to analyze modern carbonate accumulations in terms of geographic, as well as
tectonic, setting and to apply to them the tectonic classification presented above
for ancient buildups. This is done to emphasize the problems of using recent
analogs and models in our interpretative thinking about ancient carbonates.
1. Basinal buildups; areas of great subsidence.
a) Offshore banks with oceanic influence. (No modern banks oflarge size exist in marginal
cratonic basins.) E.g., Bahama Banks.
b) Geosynclinal and oceanic volcanic areas. E.g., some Pacific atolls.
2. Major platforms and ramps developed off cratonic blocks or as fringes off the same, areas
of great subsidence; all under oceanic influences.
a) Reef-rimmed coastlines with lagoons (barriers) or reef-rims directly against coasts
(fringes). E.g., British Honduras and Great Barrier Reef, Australia.
b) Barrier island rimmed coastline with remnant Pleistocene barriers as well as those
formed of Holocene sands. E.g., northeastern Yucatan coast; Shark Bay, Western Aus-
tralia; Florida Keys and Straits.
3. Buildups within platforms; areas of moderate subsidence.
a) Shelves presumably on ramps built out into shallow marginal cratonic or foredeep
basins with scattered patch or pinnacle reefs. E.g., Persian Gulf (specifically the Great
Pearl Bank). This is the single example of a land-locked epicontinental sea or foredeep
basin existing in a carbonate-producing realm.
b) Shelves on continental margins drowned by recent sea-level rise with only relict sedi-
ment. Some scattered outer reef knolls. E.g., Sahul, Campeche, West Florida, Nicaragua