Geoscience Reference
In-Depth Information
Table 6.5
Continued
Processes/ features
Possible environments
Ooids, peloids, abundant bioclasts
or bioherm/biostrome
Part of a carbonate platform. Consider position and distribution of
different facies to determine the type of carbonate platform (ramp,
rimmed shelf, isolated, etc.)
Sedimentary deposits of highly
variable composition
Consider submarine and alluvial fans, glacial environments
6
Ice features (e.g. dropstones, glacial
striations, hummocks, eskers)
Glacial environments
Large-scale lensoid sand bodies
encased in fi ne-grained deposits
Meandering fl uvial system or part of a submarine fan complex
Worked Example 6.2 Meandering river depositional environment: Burniston,
Yorkshire, UK
Figure 6.12 shows the interbedded mudstones
and sandstones exposed near Burniston,
Yorkshire, UK. These sedimentary rocks form
part of the Long Nab Member and are Middle
Jurassic in age. The sea-cliffs and foreshore
together provide an opportunity to record the
three-dimensional nature of these laterally variable
sedimentary deposits. Many publications have
been written on these deposits and the material
presented here shows only some of the fi ndings.
The research that was conducted on these
deposits was of particular interest not only for
understanding the architecture of a meandering
river system, but also because it provided an
analogue for petroleum reservoir rocks in the
North Sea, offshore UK.
lensoid sandstone body (the river channel fi ll)
encased in fl oodplain mudstones. The sand to
clay ratio will vary with the energy of the river,
sediment source and its proximity to the centre
of the channel. Figure 6.12b shows a tabular
cross-stratifi ed quartz arenite that is interpreted
to have been deposited as a subaqueous sand bar
near the centre of the fl uvial channel. Figures
6.12c and 6.12d show the lensoid point bar that
is laterally equivalent to this channel sand bar.
The succession also contains thin, fairly laterally
continuous, sandstones with rootlets (Figures
6.12e). This distinctly different sandstone facies
represents crevasse-splay deposits. Some of these
sandstones have gutter casts at the base (Figure
6.12f) from which palaeocurrent measurements
could be taken to ascertain the orientation of the
channel. Figure 6.12g shows an abandoned
channel fi lled with plant-rich mudstones. It
probably represents the sediment fi ll of a small
ox-bow lake.
The deposits at this exposure require a
combination of different techniques because of
their variable nature. These include the
construction of sets of graphic logs with the
positions of the logs carefully chosen so that
they recorded the overall variability, mapping of
the geometry of the sandstone bodies using aerial
photographs of the foreshore at low tide (Figure
6.13) and sketching the geometry of the sand
bodies. From these observations a facies model
could be constructed (Figure 6.14).
In addition to the sedimentological interest the
low-energy overbank environment provided ideal
conditions for the preservation of plant fossils.
Plant fossils from these deposites have been used
to reconstruct the Jurassic vegetation at that time
and to gain an understanding of the climatic
regime (Figure 5.14, p. 98).
Figure 6.12a illustrates a typical view of these
(and other) meandering river deposits with a
