. Bulletin of the British Museum (Natural History), Geology. BENTHIC LUDLOW COMMUNITIES 199 Sphaerirhynchia and Isorthis, while Grid K, 2 m away, is dominated by iCamarotoechia\ Salopina, bivalves and ostracods. This small-scale variation may indicate that transport during formation of the shell layer was limited, partially maintaining original distributions. It probably does not result from different hydrodynamic properties of the various taxa, which would be nearly identical in the case of Isorthis and Salopina. ] OTHERS HI BIVALVES EACH LETTERED GRID = 100 CM2 A^\ r\B surface , C STRlKE= N3
. Bulletin of the British Museum (Natural History), Geology. BENTHIC LUDLOW COMMUNITIES 199 Sphaerirhynchia and Isorthis, while Grid K, 2 m away, is dominated by iCamarotoechia\ Salopina, bivalves and ostracods. This small-scale variation may indicate that transport during formation of the shell layer was limited, partially maintaining original distributions. It probably does not result from different hydrodynamic properties of the various taxa, which would be nearly identical in the case of Isorthis and Salopina. ] OTHERS HI BIVALVES EACH LETTERED GRID = 100 CM2 A^\ r\B surface , C STRlKE= N37°E of bedding pit me iik Up ^ 5-|ow O H o i i i 50 CM I I m\ PROTOCHONETES IM LUDLOVIENSIS as&ss II °o°°°o mini 6,To °°o° 3°°°' ft . + + + + + + rl~£"= 0 li E F Ills ttt â TT H o 00 o°o° âoo° qOqO °o°° o 2 oo o o o G H K tSl" MMrn rm SALOPINA LLU L UN ATA 50- 0 J NAVICULA FJ\ ISORTHIS C-l ORBICULARIS SPHAERIRHYNCHIA WILSONI U3 38 28 38 61 33 CI 16 32 U0 37 1*07 En. lip HYaTTIDINA CA/V/)L/S 'CAMAROTOECHIA' NUCULA number of shells â¡ shells inclined to bedding surface concave upward shells Fig. 8 Faunal variation in a very thin 'swell-lag' layer of transported shells, which covers the entire bedding plane and not just the sampled grids. This example is from the Sphaerirhynchia wilsoni Association of the Lower Leintwardine Beds, section 4A, and is shown in Fig. 10 as sample 4A45. The swell lag mechanism discussed by Brenner & Davies (1973 : fig. 12) is a probable cause of the thin shell layers like that described above. Swell lags may form in fine-grained shelf sediment when storm-produced waves rework the upper shelf surface, suspending shells and sediment grains. Shells then settle first as a concentrated, coarse fraction, without significant lateral move- ment. Storm-deposited beds of transported shells in the coquinoid siltstone facies have been described in the previous section, and are shown in PI.
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