The sedimentology and palaeoecology of the Coralline Crag (Pliocene) of Suffolk

Balson, Peter Steven (1981) The sedimentology and palaeoecology of the Coralline Crag (Pliocene) of Suffolk. Doctoral thesis, Polytechnic of North London.


The Coralline Crag is a Pliocene, shallow marine, bioclastic sand formation with an outcrop restricted to south-east Suffolk. Four distinct sedimentary facies have been recognised which indicate that the Coralline Crag was deposited during a single marine transgression. The stratigraphically lowest facies is a nodular phosphorite horizon resting on an eroded surface of London Clay (Eocene). The components of the phosphorite deposit include phosphatic pebbles and cobbles and assorted phosphatised fossils which were derived from a variety of sources including a Miocene sand formation which was broken up during the transgression. The composition of the phosphorite mineral has been examined using electron microprobe, X-ray diffraction and X-ray fluorescence techniques and found to be francolite (carbonate fluorapatite). The phosphorite is overlain by a bioturbated, bioclastic silty sand facies deposited in shallow marine conditions at an early stage during the transgression. This facies is succeeded laterally and is overstepped by a sandwave facies of cross-stratified comminuted and abraded bioclastic sand. The sandwave facies is succeeded laterally north and eastwards by a facies of coarse skeletal sands with a relatively small terrigenous sediment content.

The sedimentary and faunal characteristics of these facies and their vertical and lateral distribution indicate the former presence of a linear sandbank (sandwave facies) possibly parallel to the Pliocene shoreline. Sediment travelled along a transport path from the north-east, along the bank and was deposited in the nearshore zone.

Subsequent meteoric diagenesis led to widespread dissolution of aragonitic grains. Dissolution was restricted to facies with high primary porosity which were preferentially preserved from erosion by the concomitant cementation. Calcitic bryozoans are proved to be useful environmental indicators when studied in conjunction with sedimentary evidence. The unusual large cyclostome Bryozoa characteristic of the Coralline Crag, are shown to have varying growth morphology dependant on substrate type.

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