Geoscience Reference
In-Depth Information
channels (Bridge & Gordon 1985). Bankfull dis-
charges calculated at four cross-sections, thought
to be within about 10 km of the shoreline, ranged
from 40 to 115 m
3
s
21
. Although similar small
rivers are recognized in eastern Pennsylvania
(Sevon 1985), by the time the coastal plain had
prograded through central Pennsylvania the rivers
were fewer in number and larger in dimension
(Rahmanian 1979; Williams 1985). The low palaeo-
latitude (less than 208) resulted in a tropical climate
with alternating wet and dry seasons along the
southern edge of the Euramerican landmass
(Woodrow & Sevon 1985).
avulsion site and prograding down-current as
additional sediment is transported and deposited at
the margins. Intense alluviation of the floodplain is
fuelled by the large drop in energy as the system
evolves from a single channelized flow into
rapidly evolving distributary channels of the alluvial
wedge. These channels, in turn, debouche into
waters ponded on the floodplain, the result of pre-
existing channel levees and the high friction of
floodplain vegetation (Fig. 3). Deposition proceeds
by basinward extension of coalescing splays and
lacustrine deltas fed by anabranching networks of
distributary channels. The splays and deltas build
into the transient lakes created by flooding due to
the avulsion. In the process of progradation, new
channels form by crevassing and bifurcation at
channel mouths, and others lengthen by basinward
extension. Both serve to deliver new sediment to
the flooded basin so that further progradation can
continue. Deposits of this stage are commonly: (1)
coarser-grained crevasse splays assuming a variety
of lobate, elliptical or elongate shapes and usually
containing multiple and variously sized distributary
channels that route water and sediment to and
beyond the splay margins (O'Brien & Wells 1986;
Smith 1986; Bristow 1999); and (2) finer-grained
lake and distal splay deposits in which rapid burial
has preserved organic debris from oxidation.
Stage II of the avulsion cycle is marked by distri-
butary channels that begin to flow sub-parallel to the
Depositional model
Traditional views of sedimentation in upper alluvial
and coastal plain settings envision a single-thread
meandering river continually feeding fine-grained
sediment to a slowly aggrading floodplain as the
alluvial ridge accumulates coarser-grained sedi-
ment. However, recent studies of modern
fine-grained fluvial systems that are experiencing
avulsions show that these systems cycle through
two stages with a typical period of the order 1000
years (Smith et al. 1989; Slingerland & Smith
2004; Soong & Zhao 1994).
Stage I begins when a channel changes course by
permanently breaching its levee. Here, a sediment
wedge is constructed, headed at
(or near)
the
Fig. 3. Depositional environments during Stage I of the avulsion model envisioned for Red Hill sedimentation.
Watercolour fromCumberland Marshes of the Saskatchewan River, SK, Canada. Evolutionary innovations described in
the text are thought to have arisen in a similar terrestrial setting.
