SEDIMENT GRAVITY FLOW DEPOSITS FROM NEOPROTEROZOIC
MIXED CARBONATE-SILICICLASTIC SUCCESSION OF LESSER
HIMALAYA; ARE THEY HYBRID EVENT BEDS (HEB)?
Ajay Kumar Maurya*, Subhojit Saha
Sedimentology Group, Wadia Institute of Himalayan Geology, Dehradun
Email: ajay@wihg.res.in
The classical ‘Bouma sequence’ seldom fails to explain the origin of co-occurring sediment gravity
flow and transitional flow deposits within many turbidite systems. These types of deposits are
variably mud rich, encountered in the marginal parts of the shallow to deep water systems and
variably named as slurry flows, cogenetic turbidite-debrite beds, linked debrite, matrix rich beds
and hybrid event beds etc. However, many outcrop based and experimental published models have
portrayed the close association of turbidite, transitional flow deposits and debrite as part of the
same event. So far the postulated mechanisms are i) run-out of a debris flow following partial
transformation to a forerunning turbidity current, ii) forced flow transformation due to turbulence
dampening by bulking of clay and deposition of sand-mud together and iii) segregation of sand
from suspension before the onset of cohesion and en masse deposition of sands and clay for the
remainder of the bed. Therefore irrespective of the governing mechanism flow transformation is
key process for the deposition of these hybrid (mixed) beds.
In the recent time, due to distal occurrence and hydrocarbon potential these classes of deposits are
increasingly recognized and grossly termed as Hybrid event beds (HEBs). The ideal succession of
HEBs reflects changes in flow rheology, flow pattern and other important factors related to fluid
flow and comprises five major divisions (from base to top) i.e., H1 (massive clean sandstone;
product of high density turbidity current), H2 (Banded Sandstone; deposited in transitional flow
condition), H3 (debris flow), H4 (Sand-mud couplet; resulted from low density turbidity current)
and H5 (Mud; Suspension Fallout). However, all five division may or may not be present in all
HEB like H2 and H4 may be poorly developed or dominating in the succession. Interestingly a
scan of literature portray that the HEBs are reported mostly from the Mesozoic, Paleozoic and/or
recent times. Documentation of the HEBs from ancient deposits, in particular, Archean and
Proterozoic is very rare. Since it is very common in distal reaches of the delta and deeper water
environments it can used as a stratigraphic marker for Paleogeographic reconstruction, which is
difficult in ancient sediments due to unavailability of fossils, interbedded volcanic/volcani-clastics
and post depositional deformation. Therefore in the present study an attempt has been made to
characterize the sediment gravity flow deposits from the Neoproterozoic mixed carbonatesiliciclastic packages (i.e. Mandhali Formation) of Garhwal Lesser Himalaya on the basis of
outcrop and petrographic studies to understand its development.
Keywords: Sediment Gravity flow, HEB, Neoproterozoic, Lesser Himalaya.