Climatic cycles influence the cyclostratigraphy of deepwater sediment
and are often tied to planetary orbital parameters that vary:
Discussion of
source terrains, controls and setting
Deepwater mass movement sediments range form sand to mud prone and
differences in their character are tied to the sediments eroded
from their source terrain and the morphology of the depositional
setting. This has resulted in Reading and Richards (1994) devising
a classification scheme, seen in the cartoon above, for deepwater
turbidite systems based on grain size and type of feeder system
that further contrasts fine-grained and coarse-grained turbidite
systems. The figure and table above divide these into the end members
of fine grained turbidites systems and sand rich systems. To this
end Stelting et al, (2000) have defined fine grained deepwater sediments
as those that are composed of shale and/or silty mudstones with
an equal or lesser thickness of sand-rich beds for that particular
turbidite system. Reading and Richards (1994) believe that if these
finer sediments comprise at least 70% of the total succession, the
system should be referred to as being "mud-rich". In contrast
"sand-rich" systems are those that have a high net-to-gross
ratio of sand and the shale accumulations are much thinner than
the sand-dominated successions (Bouma, 2000).
Stelting et al, (2000)
have formalized how fine-grained, mud-rich turbidite systems are
connected to large basins on passive margins. Sediment accumulation
in these basins is associated with long terrestrial transport distances,
a broad shelf, and deltas that provide sediment to that shelf. Sediment
transport across the basin is efficient, driven by slope, tectonic
movement and sea level change, so that a fair amount of the sand
bypasses the fan to reach the outer fan, though net-to-gross ratio
patterns vary across the fan. In contrast Stelting et al, (2000)
recognize that coarse-grained, sand-rich turbidite systems are associated
with smaller basins on continental crust with shorter terrestrial
transport distances, a narrower shelves, canyon-sourcing, a nonefficient
basin transportation system, a progradational depositional style,
and a net-to-gross percentage that decreases laterally from the
sediment transport pathway.
These observations match
those of Nilson et al, (2006) who summarize how turbidite and other
non-cohesive density flow sediments and mass-transport deposits
(MTD) accumulate on different continental margins as functions of
local and global controls.
It would appear that
MTDs are favored by excessive local sediment sequestration within
the adjacent shelf accompanied by slope failure. This is exemplified
the character of the shelf margin in the Bering Sea. This area acted
as a sink to glacial and volcanic sediments behaviour has been modulated
by an active tectonic substrate and glacial eustasy. The result
is a margin dominated by numerous giant, gullied canyons that act
as conduits for MTDs with few associated turbidite systems (Nilson
et al, 2006).
In contrast the Cascadia
and San Andreas margins off North America have less sediment sequestrationso
with the result that turbidites and other non-cohesive density flow
sediments are the dominant deposits. In other areas where the tectonic
and sedimentary history is variable, as in the vicinity of the Ebro
deltaic complex of Spain, portions of the basin margin favor non-cohesive
flow sediments alone while other segments only have aprons composed
of MTDs (Nilson et al, 2006).
Large delta-fed passive
margins commonly are associated with inter-mixed large and small
scale MTDs and non-cohesive density flow sediments like turbidites.
The Mississippi and Amazon provide examples of this, with their
stratigraphic histories of varying rates of sediment supply. In
the case of the Mississippi Fan the character of the intraslope
mini-basins is additionally tied to salt tectonics. Large-scale
up slope failures in conjunction with multiple failures of canyon
and mini-basin walls fill the salt mini basins with sheets of MTD
beds that are interbedded and also cover many associated fan channel-levee
complexes (Nilson et al, 2006).
Climatic events, particularly
those related to glaciation, explain the massive MTDs of some the
Antarctic margins, and the mixed MTDs and turbidites of others (Nilson
et al, 2006). Also climatically driven lower sea levels in regions
with temperate climates commonly produce lowstand MTDs (Nilson et
al, 2006).
References Cited
Nilsen, Tor H., Gary S. Steffens, and Joseph J. R. Studlick, 2006,
"Mass Transport Deposits in Deepwater Outcrops: Depositional
Setting(s), Types, and Recognition", SEPM Research
Symposium: The Significance of Mass Transport Deposits in Deepwater
Environments II, AAPG Annual Convention, April 9-12, 2006 Technical
Program
Reading, H. G., & Richards, M. (1994). "Turbidite
systems in deep-water basin margins classified by grain size and
feeder system". Bull. Am. Ass. Petrol. Geol., 78,
792-822.
Stelting, C.E., A.H. Bouma, and C.G. Stone, 2000, Fine-Grained
Turbidite Systems: Overview, in Fine-Grained Turbidite
Systems/edited by A. H. Bouma and C. G. Stone. AAPG Memoir 72, SEPM
Special Publication No. 68., p1-8
Stow, D.A.V., 1994. "Deep-sea processes of sediment
transport and deposition. In: Sediment Transport and Depositional
Processes", ed. by K. Pye, Blackwell Sci. Publ. pp
257-293.
Stow, D.A.V. and Mayall, M., editors, 2000. Deep-water Sedimentary
Systems: Thematic Set, Marine and Petroleum Geology, Volume 17,
No. 2.
USGS
Coastal & Marine Geology Program Offices provide
a number of extensive surveys of the continental margin of the USA.
