Geology Reference
In-Depth Information
12 Recognizing Paleoenvironmental Conditions
This chapter provides an overview of microfacies data
that assists in recognizing environmental parameters
that control carbonate deposition and the distribution
of organisms. Section 12.1 summarizes paleoenviron-
mental proxies. Specific attention is given to paleocli-
mate-indicative criteria (Sect. 12.2) and to paleo-depth
criteria (Sect. 12.3). The last section of this chapter deals
with microfacies data that allow seismic events to be
recognized.
Prime marine environmental parameters are nutri-
ent supply, salinity, water energy, oxygen, turbidity, sub-
strate, rate of sedimentation, water depths, light and
temperature. Modern world oceans are characterized
by three all-important ecological gradients: (a) depth
gradient; light intensity, temperature and oxygenation
decreasing with increasing water depth, (b) shore to
open ocean gradients, e.g. nutrient levels decreasing
from inland waters to the open ocean, and (c) latitudi-
nal gradients, e.g. higher productivity at low latitudes.
These gradients are commonly modified by local and
regional controls as well as changes during time. The
reconstruction of carbonate paleoenvironments and the
conditions that controlled deposition and lithification
need a critical and combined assessment of paleonto-
logical, sedimentological and geochemical data.
composition and energy level) to fossil assemblages.
•
An integrated approach, which combines different
methods and uses information from the sedimentary
record including microfacies.
Most microfacies criteria aimed at a discrimination
of ancient environments are qualitative. They refer to
the (1) modes of composition, degradation and preser-
vation of carbonate grains (e.g. growth continuities and
discontinuities, coating, worning, breaking, rounding),
(2) textural differences (grain size, packing, sorting,
maturity level) as well as (3) homogenization, biotur-
bation and bio-retexturing of the sediment.
Limitations to paleoenvironmental reconstructions
concern the interplay of environmental parameters and
environmental condensation.
The latter term describes
the projection of two or more environments onto a single
stratigraphic level during times of reduced or missing
sedimentation. The time involved ranges from several
months and years to hundreds or thousands of years.
Replacement of one environment by another will cause
autochthonous mixing of different communities, spe-
cifically in shallow shelf environments. Environmen-
tal condensation may be recognized by sedimentologi-
cal and taphonomic criteria as well as the principle of
ecological incompatibility (Fürsich and Aberhan 1990;
Kidwell 1991).
The paleoenvironment is the sum of non-biogenic
parameters that provide the framework of where an-
cient organisms lived. Because strong interactions ex-
ist between organisms and their environment, fossils
are the most important indicators of ancient environ-
ments. The interpretation of paleoenvironments rests
strongly on palecologic analyses. Several texts, listed
below, deal with the palecologic approach and also pro-
vide information on relevant proxies which will be
briefly referred to in the following sections.
12.1 Reconstructing Environmental
Constraints
Methods for paleoenvironmental reconstructions are:
•
An actualistic approach, which compares fossil
organisms with Recent relatives.
•
The geochemical approach that uses information
locked in the isotopic or trace element record of bio-
genic hardparts (Sect. 13.2).
•
The functional approach, which interprets the mor-
phology of biogenic hardparts according to the real
or inferred function of the skeleton.
Basics: Paleoecology and environmental proxies
Bosence, D.W.J., Allison, P.A. (eds., 1995): Marine paleoen-
vironmental analysis from fossils. - Geological Society
of London, Special Publication,
83
, 272 pp.
Boucot, A. (1981): Principles of benthic marine paleoecol-
ogy. - 463 pp., London (Academic Press)
•
An ecological principle approach, which applies ba-
sic ecological concepts (e.g. the relationship between
species diversity and the degree of environmental
stress; or the relationship between trophic group
