Lower Sinemurian foraminifera and Ostracoda from two fissure deposits in the Eastern Mendips (Somerset, England)

Foraminifera and Ostracoda are recorded from two Mesozoic fissures in the Eastern Mendips (Cloford and Holwell). The microfaunas are indicative of the Early Sinemurian (bucklandi–semicostatum Zones), and provide the first evidence for the former presence of sediments of this age on the Mendip “swell”.


INTRODUCTION
T h e Carboniferous Limestone of the Mendips-Glamorgan region is unique in containing numerous vertical fissures filled with a complex array of sediments ranging in age from Rhaetian to Bathonian. T h e fissures are generally truncated by the pre-Inferior Oolite (Bajocian) unconformity surface ( Fig. 1)l. Published accounts (e.g. Moore, 1867;Robinson, 1957;Savage & Waldman, 1966) have concentrated on thlose fissures with Rhaetian sedimentary infills which contain a diversity of early mammal remains unknown from other types of deposit. Recent research largely b:y Mr. C.T.J. Copp, however, has shown that many fissures contain ? Hettangian, Upper Sinemurian, Plicnsbachian and also rare Toarcian (Junction Bed) and possibly Bathonian (Fuller's Earth) marine sediments.
The fissures are considered (C.T.J. Copp, pers. comm.) to represent old joint planes, or less frequendy, faults (as i n the case of Holwell fissure HBVII) which opened up during phases of intra-Early Mesozoic folding. T h e arrangement and orientation of the fissure lithologies suggest that the sediments were sucked down from their bedded position upon the top of the Carboniferous Limestone which formed the sea floor. Most o f the fissure sediments arc thus not neptunian dykes (i.e. passive openings in the sea floor, gradually filling with in situ sediment) as previously thiought (e.g. Robinson, 1957), but are much more dynamic and tectonically controlled. T h e major fissure:, (c.g. thosc \tudied here) have an east-wcst trend parallel to the axial trend of the Mendip pericline, while smaller fissures tend to follow the regional joint pattern.
Two siltstones from separate fissures at Cloford and Holwell were sampled by the author in an attempt to obtain microfaunal evidence for their age. The microfaunas are described in this paper. Macrofaunas from the fissures are generally very sparse with ammonites occurring only very rarely, so that any evidence of age is vii 1 ua ble .
The sampling localities are as follows:

data]) and
Ogmoconcha hugrrro wi Drexler (planorbis-mid turneri Zoncs). The age interpretation for this ostracod species association hinges on the upper range limit of 0. ellipsoidea. Although present up to the semicostatum Zone (see above), the species is only common as high as the bucklandi Zone (as at Hock Cliff and Wilkesley). The Ostracoda thus support the hucklandi Zone age suggested by the foraminifera but are not precisely confirmative. The known British ranges and total ranges of all the taxa recovered are shown in Table 1.

DISCUSSION
The two assemblages from similar siltstone lithologies suggest analogous ages of bucklandi-semicostatum Z o n e s C a r b o n if e r o u s 0 s t r a co d a ) (Cloford) and bucklandi Zone (Holwell). In view o f the dominantly tectonic mode of emplacement of the fissure deposits, the present study provides the first evidence of the former presence of Lower Sinemurian (bucklandisemicostatum Zone) sediments on the Mendip positive area or "swell". The nearest coeval bedded rocks arc to the west at Shepton Mallet where they occur in littoral and semi-littoral facies (Moore, 1867) quickly passing to the south into normal offshore Blue Lias facies (e.g. at Cannard's Grave; Donovan, 1958   are also recorded Rhaetian sediment, grey bioclastic limestone (probably Late Sinemurian on non-ammonite macrofossil evidence) and greenish clay of possible I'liensbachian age (C.T.J. Copp, MS). The Cloford fissure (CXIV) is known to contain, in addition to the Lower Sinemurian siltstone described above, limestones of probable jamesoni Zone age.