LAYERING - WHAT DOES IT MEAN?

Robert Ginsburg made that passing comment to me in the mid-1960s. It was timely and has crossed my path and that of students and colleagues many times since. We have a few answers for you.

Working with you on Andros, we monitored tidal flat sedimentation and learned that winter storms events generated laminated carbonates and that specific laminae form was influenced by cyanobacterial mats. Mat type was a function of ‘exposure Index' or, as learned on Caicos, frequency of sedimentation events. Caicos has a carbonate tidal flat dominated by cm-thick beds from hurricane events. Grain size permitted differentiation of tidal-flat beds from adjacent marine cm-thick beds. Sediment swept onto the flat by hurricanes was suspension transported – less than 200 microns. Offshore layers contained all grain sizes. Bagnold's boundary between bedload and suspension transport helps explain the dynamics of layering throughout the modern and ancient.

Many muddy carbonate sequences once thought to be biogenic are now recognized to be of physical, transported origin. Layering is better preserved where individual depositional units are thicker, coarser grained, or happen frequently (e.g. tidal or river deltas within lagoons) and are a proxy for layering processes adjacent. When bioturbation has destroyed primary stratification, constituent grains can help reconstruct primary layering origin and processes.

As illustrated in dune ridges in the Bahamas or coastal marl ridges in south Florida, pulses of rapid sedimentation occur following small sea level or sediment supply changes, these are preserved well layered in an environment subsequently vegetated, stabilized and inactive. Pulses may be more common in building a stratigraphic record than we presently appreciate.

Sedimentation events are commonly best preserved when not deposited on the sediment surface but rather below (tubular tempestites). This has been useful in re-evaluating depositional environments, such as Paleozoic mud mounds.

Selective diagenesis (cementation, recrystallization, dolomitization, and pressure dissolution) commonly accentuates subtle differences in primary layers that would not otherwise stand out. In some cases burial diagenesis can create false layering in the same manner as tectonic stress creates spaced solution cleavage.