Geology Reference
In-Depth Information
Layering: what does it mean?
HAROLD R. WANLESS
Department of Geological Sciences, University of Miami, P.O. Box 249176, Coral Gables, FL 33124, USA
(E-mail: hwanless@miami.edu)
ABSTRACT
'Layering. What does it mean?' Robert Ginsburg made this passing comment in the
mid-1960s when the author was looking at a core sequence from Biscayne Bay, Florida.
The core was a burrowed carbonate wackestone with a Hurricane Betsy storm layer
at the top. It was a timely comment as the question has been asked many times since
then. While a few important answers have emerged, there are still some serious chal-
lenges ahead. Understanding layering is more than just visually looking at the layers.
It includes an appreciation of grain size, sorting and abrasion, infl uences of bioturba-
tion and subsequent diagenesis. Together, these provide the critical tools to recon-
struct palaeoenvironmental setting, defi ne depositional processes and determine the
spectrum of energy levels operating in the system.
Keywords
Layering, texture, burrowing, diagenesis, Ginsburg.
PRIMARY STRATIFICATION
tend to be built of centimetre-thick bedding
not millimetre laminae. These are fl ats that are
seldom infl uenced by winter-storm events but rely
on hurricanes to build their sequence. Centimetre-
thick bedding occurs on the inner marsh on
Andros where the cyanobacterial mats were
punctuated by thicker sediment layers from
hurricanes, but not reached by winter storms.
Based on fi eld observations, channelled carbonate
fl ats subjected to winter storms build laminated
levees, and fl ats or portions of fl ats affected only
by larger hurricane events produce centimetre-
thick bedding.
The question of layering was actively pursued
by Robert Ginsburg, Laurie Hardie and others on
the tidal fl ats on Andros Island in the Bahamas.
These studies documented that laminated carbon-
ates formed on the tidal fl at supratidal channel
levees because of winter storm fl ooding events.
Layering thickened on the lower levee backslope
approaching the pond, but grazing and infaunal
bioturbation quickly destroyed deposited lay-
ers (Hardie, 1977). It was a revelation that not
all laminated carbonates were deep basinal. This
work caused a sudden trend for reinterpretations
of laminated carbonates. Many laminated carbon-
ates interpreted as deep basinal were now realized
to be shallow tidal fl at (and of course, some that
were in fact deep basinal were misinterpreted to
be tidal fl at).
The specifi c form of the laminae produced (fl at,
crinkly and such) was the result of cyanobac-
terial mats growing on the surface (Fig. 1), and the
distribution of the different mat types was a
function of 'Exposure Index' (the inverse of
the frequency of fl ooding; Hardie, 1977) or, as
later documented on Caicos, the frequency of
sedimentation events (Wanless
et al.
, 1988b).
Working on Caicos, and with the help of
Hurricane Kate in 1985, Wanless
et al
. (1988b)
demonstrated that some carbonate tidal fl ats
GRAIN SIZE
This centimetre-thick supratidal bedding described
above presented a problem because it was now
recognized that hurricanes produce centimetre-
thick stratifi cation in both normally subtidal and
normally supratidal environments. These can be
differentiated by grain size. The sediment swept
across a tidal fl at by Hurricane Kate was that which
could be carried in suspension. In contrast, the off-
shore storm layer contained a total mix of coarse
to fi ne grain sizes moved around the offshore bot-
tom by the storm (Wanless
et al
., 1988b). Bagnold
(1966), working on desert sands and in rivers in
Africa, and McCave (1970, 1971), working on sands
