Palaeoeology sedimentology and depositional environments of the middle lias of North Yorkshire.
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The shallow marine Yorkshire Middle Lias (Lower Jurassic) sequence
comprises two formations, the lowermost (Staithes Formation) consisting
of interbedded bioturbated siltstones and thin sheet sandstones and
the uppermost (cleveland Ironstone Formation) comprising bioturbated
shales, siltstones and Minette-type oolitic ironstones. Two new subdivisions,
the Penny Nab and Old Nab Members, are proposed for the
Cleveland Ironstone Formation.
Lithofacies distributions during Staithes Formation times reflected
the gradual progradation, retreat and transgressive reworking of a
barrier island complex. Sand was eroded from the barriers by powerful
storm waves and entrained along narrow paths further out on the shelf
by offshore flowing storm surge ebb currents. Temporal changes in
these paths were mainly responsible for the vertical lithofacies
transitions within the sequence.
In early Cleveland Ironstone Formation times four phases of
shoreline progradation were each terminated by transgression induced
sedimentation breaks, leading to the deposition of four shale -
silt stone - ironstone "punctuated aggradational cycles". During these
breaks and throughout late Cleveland Ironstone Formation times
there was a drastic reduction in terrigenous siliciclastic influx
into the basin of deposition but concomitantly increased input of
colloidal reduced iron compounds. Ironstones formed by nearshore
flocculation and concentration of iron hydroxides and their reaction
with detrital kaolinite (within the guts of benthic marine organisms)
to form chamosite, which was subsequently incorporated into ooliths.
Residual iron hydroxides were converted to siderite during early
burial diagenesis.
Both quantitative and qualitative analysis of the macrofauna
and ichnofauna has led to the recognition of fourteen recurrent nonrandom
"associations" of body and trace fossils. These represent the
incompletely preserved, biased but in situ relics of Middle Lias
benthic comunities. Their compositions and distributions were mainly
controlled by substrate grain size and consistency, the availability
of microsubstrates for attachment purposes, food availability and
bottom water turbidity and dissolved oxygen content.
Authors
Howard, Andrew StephenCollections
- Theses [3834]