Page 67 - Carbonate Facies in Geologic History

54 The Stratigraphy of Carbonate Deposits

1. Marine transgressions must have occurred over extremely level surfaces
formed by a preceding regression associated with construction and seaward mi-
gration of a flat coastal plain.
2. Any kind of independent sea-level rise would "rapidly" flood wide areas of
such surfaces, especially when they are tectonically subsiding. But, although ma-
rine transgression occurs very "rapidly," wave action to erode the substrate is
dampened by the very low relief over such wide areas. Marine transgression
naturally cuts off sediment supply by causing upstream aggradation, decreases
sediment influx, and so gradually increases its own progress. Transgression is apt
to appear more rapid than it actually is. Thin transgressive units are the rule, but
open marine shelf strata may be thin due to slower deposition and, as pointed out
by Fischer (1961), some transgressions cannibalize their own stratigraphic record.
Offshore waves beyond barrier bars and island complexes may in some instances
erode the previously deposited record as they move across the swampy inner
coastal sediments.
3. Once a typical shelf is flooded, regressive outbuilding occurs if carbonate
production is established. This happens because shallow water carbonate sedi-
mentation is an extremely rapid process and may fill in large areas in a geologi-
cally brief span of years (note figures for Holocene shallow sedimentation given in
Chapter I).
4. The rate of regressive outbuilding is, for anyone cycle, generally so rapid
that it is beyond the resolution of our biostratigraphic calendar. Sedimentary
cycles form faster than the evolution of practically any organisms upon which
paleontological zonation is based. The sharp boundaries between such cycles and
the thin transgressive record at the base represent a duration of only a few
thousand years (see below).
T~1Us, although each hemicycle or upward shoaling sequence so created fol-
lows Walther's Law, and consists in the main of regressive, diachronously depos-
ited facies, the boundaries between these sedimentary rhythms may closely ap-
proximate "time markers" and are more useful as such than the diachronous
facies within each cycle. Geologists commonly use either the contact between
cycles where inundation begins or the maximum inundation phase of an individ-
ual cycle as "approximate time lines." The principles underlying the use of such
boundaries in "time" correlation are described by Krumbein and Sloss (1963) who
term the procedure "correlation by position in the bathymetric cycle." Addition-
ally, there are certain types of strata commonly associated with transgressive-
regressive cycles which may be assumed to have formed only along flat planar
surfaces and to have been preserved by relatively rapid transgression (e.g., coals
and the top of sabkha evaporites). When sequences of such key beds lie parallel to
each other, or when their intervening beds thin or thicken at regular rates, most
geologists assume that such strata represent, for practical purposes, time-strati-
graphic markers within a given basin. Other key beds which may be used as time-
stratigraphic markers in beds on platforms or shallow basins are bentonites,
radioactive silt zones (often causing gamma ray deflections in thick carbonate
strata) and certain calcareous concretion zones in shales. These key units may be
used to check cyclothem boundaries considered as "time markers."