Geology Reference
In-Depth Information
Fig. 1.1.
Limestones as paleoclimatic proxies
. Shelf limestones formed in tropical warm-water and in non-tropical cool-
water environments differ in the association of specific grain types (see Sect. 12.2). The sample is a skeletal grainstone
consisting predominantly of bryozoans (B) and benthic foraminifera (F). Gastropods (G) are rare. Black grains are mainly
red algae. The skeletons of many bryozoan fragments have lost their structure due to the attacks of microborers and have
been transformed into microcrystalline grains. The sediment is only poorly lithified as shown by the high open porosity.
Marine carbonate cement occurs as thin rims of fibrous calcite crystals (arrows). The grain association as well as the weak
cementation are characteristic of cool-water carbonates. The sample comes from Oligocene deposits of Leg 133 of the Ocean
Drilling Program which dealt with the geological evolution of the Great Barrier Reef Province off northeastern Australia.
Courtesy of T. Brachert (Mainz).
as stable isotope geochemistry have become common
tools in sedimentological studies (Chap. 3). Chemical
analysis and X-ray techniques are well-established rou-
tine methods used to determine major, minor and trace
element concentrations and the composition of carbon-
ate rocks. The combination of these techniques with
microfacies studies offers new possibilities for the in-
terpretation of diagenetic pathways of carbonates
(Chap. 7).
Quantification of thin-section criteria, based on com-
puter-assisted frequency analysis and using multivari-
ate methods, has become an essential prerequisite for
the categorization of microfacies types, the evaluation
of the sedimentary budget of basin fills and for com-
puter simulations having the potential to constrain quan-
titative sedimentary models (see Sect. 6.3.2).
non-marine carbonate rocks. This attitude may reflect
classification problems related to the poverty in char-
acteristic non-marine microfacies types as compared
with the wealth of types known from marine environ-
ments. This hesitancy is, however, only partly justi-
fied: there exist succinct and informative reviews of
modern and ancient fresh and saltwater lake deposits,
travertines and pedogenic carbonates (see Sect. 2.4.1
and Sect. 15.1 to Sect. 15.4). Ancient calcretes, char-
acterized by a set of distinctive microfacies criteria, are
very important indicators of subaerial exposure and are
potentially very useful for defining the nature of se-
quence boundaries and recognizing changes in paleo-
climate. The same is true for paleokarst which is easily
recognized from microfacies features.
New comparative data of modern and ancient sedi-
ments.
Modern carbonate depositional systems only
partly reflect the wide range of settings and environ-
ments established during the earth's history because
biological, geological and chemical controls have
Lacustrine and terrestrial carbonates
. Despite the
increasing economic importance of non-marine sedi-
ments, many students interested in microfacies are
somewhat hesitant in applying microfacies analysis to
