Geology Reference
In-Depth Information
stratigraphic age depending on the composition of the
eroded rock sequences. Clasts occur together with iso-
lated fossils (e.g. corals or rudists transported from
reefs), or are mixed with exotic material brought in by
rivers from the hinterland. Ancient carbonate rock fall
deposits are known from near-coast and transitional
environments (e.g. Thiedig 1975), rocky coasts, sea-
mount cliffs (Seyfried 1980), and steep forereef slopes.
blocks, indicating alternations of transport and rest-
ing phases,
•
rounding of smaller and larger blocks caused by dis-
solution and biogenic encrustation,
•
occurrence of cryptic, cavity-dwelling organisms
and specific low-growing encrusters,
•
joint occurrence of blocks exhibiting different di-
agenetic histories (e.g. blocks derived from subae-
rially exposed platform parts and blocks eroded in
deep-water environments).
15.7.2.2 Breccias and Megabreccias
The development of breccias often starts with sub-
marine rockfall deposits and avalanches, followed by
the formation of megabreccias, and finally the deposi-
tion of turbidites.
Sedimentary breccias are common constituents of car-
bonate slopes (Sect. 5.3.3).
The breccias are characterized by
•
discontinuous lenses (meters to tens of meters) with
sharp and undulated boundaries at the top and the
bottom,
15.7.2.3 DebrisFlow Deposits
•
polygenic compositions, yielding shallow-water and
deep-water elements, and consisting of bioclasts and
lithoclasts,
Debris flows composed of clasts and supported and car-
ried by a mud-water mixture lead to the deposition of
sediments that have variously been called debrites, de-
bris sheets or mass breccia flows (Hiscott and James
1985). Many so-called debrites have a granular matrix
that is noncohesive. In these cases, transport by turbu-
lence and grain interactions is more likely. Debris flows
are capable of traveling over very gentle slopes and
may represent a pre-phase of turbidite sedimentation.
Debrites range in thickness from a few decimeters
to several tens of meters. Most debrites form sheet-
like and lenticular bodies with conformable, sometimes
also erosional contacts with the underlying fine-grained
sediment. Upper contacts are sharp or the debrite bed
passes upward into turbidites (Krause and Oldershaw
1979).
Many debrites are coarse-grained breccias or con-
glomerates characterized by poor sorting, lack of strati-
fication and often random or chaotic clast fabrics. Clast
packing is variable and shows matrix-support as well
as clast-support. The source area of clasts can only be
inferred by comparing clast microfacies with micro-
facies of various parts of platform and slope carbon-
ates (Nebelsick et al. 2001). The common criteria of
debrites in microfacies studies are:
•
•
variously shaped and usually poorly sorted clasts,
•
clasts floating within a fine-grained matrix (com-
posed partly of comminuted breccia material) or ex-
hibiting fitted contacts,
•
clasts sometimes showing a preferred orientation of
longer clasts.
Megabreccias
are matrix-supported breccias consist-
ing of well-defined blocks, from meters to more than
several hundred meters in size. Seismic shocks and
gravity collapses of the outer parts of high-angle outer
carbonate platforms may contribute to the development
of huge breccia masses deposited in upper and mid-
slope settings. Scalloped embayments and large-scale
erosion features depicted in seismic reflection profiles
record the catastrophic removal of many cubic kilo-
meters of sedimentary units and explain the presence
of major megabreccia sheets at the toes of slopes
(Mullins et al. 1986; Mullins and Hine 1989; Hine et
al. 1992; Stewarts et al. 1993; Sano and Tamada 1994).
An excellent overview of the importance of mega-
breccias in the context of sequence stratigraphy was
provided by Spence and Tucker (1997). Common fea-
tures of megabreccias are the
•
Sedimentary units are generally massive or coarse-
bedded, often with significantly thick beds. Irregu-
lar top surface.
close association of breccias with various types of
mass-flow deposits,
•
occurrence of megablocks consisting of reworked
limestone breccias together with breccias of vari-
able size, embedded within fine-grained matrix (see
Sect. 15.7.3.1 and Sect. 15.7.3.2).
•
Large clasts may project above from the bed.
•
No preferred depositional fabric except for crude
grading in the basal part.
•
Limestone clasts are very poorly sorted and of vari-
able size, usually of sand-grade or finer size.
•
mixture of intrabasinal and extrabasinal blocks in
size up to several hundreds of meters,
•
Angular or rounded clasts, or of mixtures of both
types occur.
•
disorientation of geopetal fabrics within individual
