Geology Reference
In-Depth Information
Fig. 15.19.
Turbidite or tempestite?
Turbidites and storm-induced tempestites may exhibit similar structures (see Fig.
15.20).This sample comes from limestone beds deposited in a base-of-slope position in a shallow inter-reef environment
close to the margins of coral reefs (Flügel 1975).
The stratigraphic section consists of bedded micritic limestones alternating with abundant intercalations of bio- and
lithoclastic beds exhibiting coarse- and fine-grained skeletal grains. The thickness of the allochthonous beds varies between
a few millimeters up to 30 centimeters. Skeletal grains were derived from shallow-water environments (dasyclad algae,
various porostromate algae; lituolid foraminifera).
The source of coral debris and shells of bivalves, gastropods, brachiopods and echinoderms were small patch reefs. The
frequency of skeletal grains varies within the allochthonous beds. Some beds yield up to 70% porostromate calcimicrobes,
some beds 40% bivalve shells and about 25% micritic intraclasts, peloids and oncoids, and other beds are predominantly
composed of oncoids, peloids and intraclasts.
The sequence shown in the figure starts with an erosional surface at the base separating the underlying micrite from the
bioclastic grainstone. The grainstone unit, consisting of bioclasts (predominantly calcimicrobes, foraminifera, mollusk shells)
peloids and some lithoclasts, is graded. Grading is reflected by the upward decrease in grain size. The upper surface of the
graded unit is sharp. The overlying sediment consists of packstone in the lower part, grading into micritic and fine-peloidal
mudstone in the upper part. Lamination is indicated by thin dark laminae and slightly thicker light laminae. Skeletal grains in
the lower part are disoriented. The tiny shell fragments occurring in the upper part of the sample have a parallel orientation.
The sample is a turbidite, as indicated by the erosive base of the graded unit (corresponding to the boundary between
Zone 0 and Zone 1a of the Meischner Sequence), the normal grading (corresponding to the transition from Zone 1a to 1b),
the differentiated lamination of the upper part (corresponding to Zone 1c and Zone 2), and the difference in the background
community that indicate the low-energy basinal depositional environment. Late Jurassic (Early Tithonian): Kapfelberg near
Kelheim, Bavaria, Germany.
