Page 102 - Geology of Carbonate Reservoirs

ROCK, TIME, AND TIME–ROCK UNITS 83

characteristics. In fact, some researchers do not make the distinction between open
shelves and distally steepened ramps. For consistency and simplicity in defi nition of

platform types, this book classifies those with slope breaks accompanied by facies
changes as shelves and those without slope breaks or with slope breaks that are not
accompanied by facies changes as ramps.

4.2 ROCK, TIME, AND TIME –ROCK UNITS

Geologists distinguish between rock units, time units, and the composite time – rock
units. Stratigraphy based on rock layers only is called lithostratigraphy. Stratigraphy

based on rock units that have corresponding time significance is chronostratigraphy.
Sequence stratigraphy incorporates time – rock units in a system of stratal architec-
ture. Time units are simply measures of geological time, usually given in millions of
years abbreviated as Ma. Time units in the geological record are ranked and named
according to the length of time they cover. Units that span the longest time are
called eons. In descending order of time span, the remaining units are eras, periods,
and epochs. The Phanerozoic Eon includes the Paleozoic, Mesozoic, and Cenozoic
Eras. Each of those eras includes periods and each period includes epochs. We live
in the Holocene Epoch of the Quaternary Period of the Cenozoic Era in the Pha-
nerozoic Eon.

4.2.1 Rock Units

Rock layers represent the three - dimensional record of earth history. Ideally, each
millimeter - scale lamina represents the sedimentary record at an instant in geological
time, but bathymetric charts of modern seas show that depositional surfaces are

rarely, if ever, flat and a single sedimentary lamina may represent a longer or shorter
time depending on rate of sedimentation and efficiency of preservation. In other

words, a lamina does not necessarily represent a time plane. Nearly every marine
environment has bathymetry that may be depositional, tectonic, or erosional in
origin. Depositional topography may result from the growth of carbonate buildups
such as reefs, mounds, or skeletal sand accumulations. Tectonic features result from
faulting and folding, and erosional topography includes scoured or karst depressions
as well as remnant hills and ridges. Submarine topography may be passively involved
with sedimentation, such as a topographic depression being filled with detrital car-

bonates, or it may be dynamically interactive, as in the case of topography being
created or accentuated by active reef growth. Depositional surfaces are not fl at and
they may change constantly as they interact with the environment. Lithogenetic
units are rock units that have a common origin and share many descriptive charac-
teristics. They may or may not be bounded by time surfaces. The practical solution
to dealing with rock units was developed in North America by Schenk and Muller
(1941) , whose pioneering concepts were incorporated in the Code of Stratigraphic
Nomenclature and the International Guide to Stratigraphic Classifi cation. Rock
units are mapped without being associated with time and they are assigned to a
hierarchy of scale that includes, from large to small, group, formation, member,
and bed. Formations are the smallest distinctively mappable rock units and they

have no time significance. They may include diachronous facies or the formations