Palaeoecology, sedimentology and depositional environments of the Middle Lias of North Yorkshire
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.