Geology Reference
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gravel- and boulder-sized lithoclasts derived from lithi-
fied platform or platform-margin carbonates. Submer-
gence of the platform may result in an increased con-
tribution of platform-margin material to slope depos-
its, leading to a higher input of lithoclasts.
Slope material: Boulders, blocks and giant carbon-
ate bodies deposited on the slope or at the toe-of-slope
are eroded from platform margins or from higher parts
of the slope. Sand- to boulder-sized carbonate litho-
clasts may also have their source in eroded rocks from
higher, lithified slope parts.
• Fine-grained shallow-marine material intermixed
with pelagic carbonate grains forms periplatform car-
bonates that can accumulate as periplatform sands at
the foot of steep platforms, as fine-grained periplatform
turbidites interbedded with pelagic oozes in basins, or
as soft periplatform ooze and mud deposited in deep
basins. The emergence of platforms shut down the
source area of platform material, and more pelagic sedi-
ment is deposited than peripelagic ooze.
Hemipelagic terrigenous material, usually clay-
sized particles, is transported across the shelf and de-
posited on slopes or in the basin. The material is inter-
mixed with carbonate sediment, forming a mixed si-
liciclastic-carbonate facies or occurs in discrete clay-
rich marly and argillaceous beds sandwiched between
fine-grained limestone beds.
Autochthonous carbonate on slopes is produced by
organisms able to induce carbonate precipitation (e.g.
microbes) or secrete calcareous skeletons (various in-
vertebrates including foraminifera, sponges, corals,
mollusks, serpulids). Some of these organisms contrib-
ute to the formation of extended reef structures, includ-
ing mud mounds, reef mounds and coral reefs.
Sediment gravity flows (mass flows) characterized
by a mixture of sediment and fluids comprise turbid-
ites, grain and debris flow deposits, breccias, and con-
glomeratic deposits.
Submarine sliding (movement of a rigid, internally
undeformed mass along a discrete shear surface) is trig-
gered by slope failures.
Downslope creeping (slumping) is a characteristic
long-term process within non- or semilithified sediment
piles, caused by bedding-parallel translocation along
well-defined surfaces or a diffuse shear zone. The sedi-
ment becomes simultaneously deformed and may de-
velop distorted bedding and synsedimentary folds (e.g.
slump folds).
The availability and volume of detrital material trans-
ported by gravity flows from neritic shelf environments
to basins depend on an interplay between the produc-
tion of biogenic material, climate, sea-level changes
and tectonics. In carbonate systems the sea-level re-
lated timing of shedding from platforms and climate is
the essential control (Droxler and Schlager 1985; Sarg
1988; Schlager et al. 1994). Climate can favor increased
volumes of specific shelf carbonates (e.g. foramol/
heterozoan carbonates during relatively cold phases)
thus providing more source material for turbidites. One
example is the Early Tertiary Zumaya series of the Gulf
of Biscay, as shown by Gawenda et al. (1999).
15.7.2.1 Submarine Rockfalls
Rockfalls are characterized by free falling, and of-
ten large rock fragments that are transported across
steep and moderate angles over short distances. The
material is eroded from cliffs, escarpments or rocky
shores (see Sect. 5.3.3.3).
Important criteria are:
15.7.2 Allochthonous Slope and Basin
Deposits: Diagnostic Criteria
Depositional units characterized by distinct bound-
aries.
Depositional processes and products: Sediment is
transported to and accumulated in slope and basin set-
tings by
Clasts are angular to subangular, uniform or poly-
mict, and usually very closely packed.
Sorting is poor to very poor. No grading.
Settling of fine-grained platform-derived material
from suspension (often forming black, very fine crys-
talline and often laminated, rhythmically bedded lime-
stones and shales).
Subaqueous rockfall and talus (e.g. below steep es-
carpments). Carbonate rockfall sediments are charac-
terized by clast- or mud-supported, poorly sorted sedi-
ments with angular clasts of different size. Rockfall
produces forereef breccia, escarpment breccias, and
various types of slope breccias.
The size of the clasts varies between a few millime-
ters to tens of meters. Large clasts occur together
with very small clasts.
The fabric is usually clast-supported (e.g. at the base
of oversteepened platform rims).
The matrix consists of poorly sorted rock debris,
fine-grained carbonate, or argillaceous material.
Microfacies: The clasts may exhibit different litholo-
gies, different microfacies types and rocks of different
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