Stratigraphic
log around Brighstone
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From Wright, 1996
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Within
the Wessex Formation there are four main lithofacies (Wright
et al., 2000), these being:
Thin
sandstones containing dinosaur footprints, caused by crevasse
splay deposits
Grey-green
mudstones containing vertebrate remains, lignite, pyrite and
siderite created in stagnating shallow ponds, similar to today's
billabongs, on the flood plain, which would act as sinks for
the debris washed in during floods.
Mottled
mudstones, containing goethite and pseudo-anticlines, created
by surface water gley soils.
Red
mudstones with pseudo-anticlines, haematite and carbonate
nodules, the lithified remains of vertisols that are formed
on elevated areas that flood only occasionally.
Most
of these lithofacies imply the presence of palaeosols, although
none have been found in their entirety in the Wessex Formation.
The mottled and pseudo-anticlined mudstones are too thick
to be natural palaeosols, as although compaction of vertisols
after burial is minimal they are thicker than normal soil
profiles, suggesting a gradual accumulation of sediment at
frequent intervals (Wright
et al., 2000).
These
lithofacies indicate a seasonal wetland, an environment common
in modern tropical and sub-tropical river systems. Whereas
a regular wetland in either permanently submerged to a shallow
depth or the ground surface is at the waterline, a
seasonal wetland has these features for a short period, and
is dry for the rest of the time. There are many modern examples,
for example the Pantanal of Brazil and the Varzea flooded
forest of the Amazon Basin. These are what are known as riparian
seasonal wetlands, associated with large rivers, influenced
by heavy rainfall to create flood effects (Wright
et al., 2000). These sudden influxes of water, and by
association, sediment at seasonal intervals would account
for the thick bedding of the palaeosols. However, the mottling
and pseudo-anticlines implies that the flooding was frequent,
quite possibly annually, with large quantities of suspended
sediment.
Stratigraphic log around Compton bay
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From Wright, 1996 |
The
mudstone facies show gradation, the green mudstones at the
bottom, the mottled mudstones on slightly higher ground and
the red mudstones are on the highest ground that is still
effected by flooding but drains easier, allowing the formation
of haematite, calcrete nodules (Wright
et al., 2000), and mud-cracks, as in the Hypsilophodon
beds at the top of the Wessex Formation.
These
lithologies imply a catena, a group of soils formed on similar
parent materials in the same climate but being different due
to drainage differences caused by topographic variation (Wright
et al., 2000).
Most
of the dinosaurs are found in the grey clay plant debris beds,
presumed to be the bottom of stagnant pools or the results
of flash flooding. Either way, the bones were buried quickly
in a reduced oxygen atmosphere, which may have lead to sulphate
reduction taking place. This would then lead to fine scale
pyrite production within the crystalline structure of the
bone, causing the bones to appear black, although the refractory
components of organic matrix of the bone may also be a factor
(Martill, 2001).
Dinosaur
footprints occur in most of the sandstone beds, with 'Iguanodon'
tracks being most common (Wright,
1996). The presence of these splay-crevasse arenaceous
layers in otherwise argillaceous strata has been attributed
to the bursting of levees (Wright
et al., 2000). It has been suggested by some that they
may be caused by short, regular, marine transgressions and
regressions (Matteson, pers. comm. 2003), although this is
not the most prevalent theory.
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