Print this pageStratigraphy of Deep-Water Turbidites, Part I: A Dynamic Geomorphological Approach
POSAMENTIER, HENRY W., WILLIAM R. MORRIS, STEVEN P. ROSS, and JAMES A. LORSONG
Deep-water turbidite systems commonly can be subdivided into three
geomorphological provinces: I) a tributive canyon (or gully) province, II) a
turbidite valley province, and III) a distributive or braided turbidite channel
complex. Reservoir facies vary predictably within each of these provinces. The
canyon segment of this tripartite arrangement serves as the conduit for sediment
gravity
flows emanating at the shelf edge and is predominantly erosional and
incised into the slope. Levees are not commonly observed in this part of the
system. Canyon fill deposits primarily comprise slump and hemipelagic sediments,
except m some active margin settings where the fill can also comprise coarse
grain material. The canyons (or gullies) feed turbidite valleys of province II
where slope gradients lessen significantly. The turbidite valleys commonly
contain multiple channel threads between the master levees. The valley fills can
be both erosional as well as depositional, though aggradational stacking
ultimately characterizes the fill. Amalgamated channelized deposits with minimal
shale breaks characterize deposits from within the turbidite valley, whereas the
levee deposits are characterized by relatively continuous interbedded shale and
thin-bedded sandstone. Turbidite valleys can range in width from less than 100 m
to greater than 8 km, and in length from a few 100 m to 100s of km. Farther
downslope, turbidite valleys feed the turbidite channel complex of province III.
This complex comprises multiple relatively small channels that spread out from
the individual leveed turbidite valley of province II. Deposits within province
III are predominantly aggradational and progradational, and are laterally
extensive due to widespread avulsions. Discontinuous shale breaks are common in
this province.
All turbidite systems are characterized by this tripartite system though the
relative dominance as well as the absolute
magnitude of each province can vary
greatly depending on a number of factors including 1) sand to mud ratio within
flows, 2) continuity vs. episodicity of flows, 3) volume 0 flows, and 4)
physiographic setting (e.g., length and gradient of slope, irregularity of sea
floor, etc.). From a sequence stratigraphic perspective, it is notable that with
the exception of the physiographic setting, each of the aforementioned factors
can vary systematically with change of sea level. Thus the sequence
stratigraphic expression, though highly variable, can nonetheless be
predictable.