. Bulletin of the British Museum (Natural History), Geology. 182 R. WATKINS these is the Upper Bringewood fauna, in which the restricted Atrypa reticularis - coral Association appears as a stratigraphic anomaly in inner shelf sections. This fauna can be accounted for as having occupied some sort of restrictive, back-barrier environment. There is only one interval in sections at the shelf edge area which includes barrier-type deposits, and this is the Upper Bringe- wood Beds containing 'banks' of Kirkidium and large corals. Synchrony of the Upper Bringewood Beds between these areas (as redefine
. Bulletin of the British Museum (Natural History), Geology. 182 R. WATKINS these is the Upper Bringewood fauna, in which the restricted Atrypa reticularis - coral Association appears as a stratigraphic anomaly in inner shelf sections. This fauna can be accounted for as having occupied some sort of restrictive, back-barrier environment. There is only one interval in sections at the shelf edge area which includes barrier-type deposits, and this is the Upper Bringe- wood Beds containing 'banks' of Kirkidium and large corals. Synchrony of the Upper Bringewood Beds between these areas (as redefined in this report) is therefore highly likely. The second synchronous ecologic phenomenon is the appearance of the distinctive fauna of the Upper Leintwardine Beds, which occupies only 1-5 m of strata in the Welsh Borderland, but occurs across the whole of the shelf area (Lawson & Straw 1956; Holland et ah 1963). This appearance involves the very short-lived colonization of proximal shelf deposits by forms from quiet-water, distal shelf communities. It can be explained ecologically only by brief, synchronous changes in shelf-wide current patterns controlling larval dispersal. Both of these ecological phenomena and their causal explanations are discussed more fully on pp. 231-237. The stratigraphic position of the Upper Bringewood Beds and Upper Leintwardine Beds is shown in Fig. 2. The Middle Elton Beds, lower in the sequence, can be considered as synchronous across the Welsh Borderland by their content of nilssoni Zone and scanicus Zone graptolites (Holland et ah 1963, 1969; Shergold & Shirley 1968). From Fig. 2, it can be seen that synchrony of these three units effectively brackets the Ludlow shelf sequence, indicating a syn- chrony for each of the units between. Sedimentology Distinction of Ludlow basin and shelf The Ludlow sediments of Wales and the Welsh Borderland form two environmental complexes, a basin facies to the west, and a shelf facies to the east (Fig. 1)
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