Horn Park Quarry, Dorset

[ST 457 021]

B.M. Cox

Introduction

Horn Park Quarry, c. 1.5 km north-west of Beaminster in Dorset, is one of the most famous Aalenian–Bajocian localities in Britain. The highly fossiliferous Inferior Oolite Formation there has been quarried intermittently over many years, and has yielded many thousands of ammonites. Sections were recorded by Richardson (1928–1930), Bomford (1948), Torrens (1969b) and Senior et al. (1970), and the succession was summarized by Parsons (1980a). Since then the locality has featured prominently in the reassessment of the Aalenian–Bajocian ammonite faunas and stratigraphy of this region (Chandler, 1982, 1996; Callomon and Chandler, 1990; Morton and Chandler, 1994; Sandoval and Chandler, 2000). It is particularly important for the Aalenian Stage as nearly all of that stage's currently recognized ammonite biohorizons are present. According to Callomon and Cope (1995), the profusion of ammonites has also given the opportunity to record the rarer elements of the fauna.

Description

The following record of the section (also (Figure 2.18)) is largely based on that of Callomon (in Callomon and Cope, 1995); his bed numbers are modified from Senior et al. (1970). The faunal determinations are biased towards the ammonites, which occur in particular abundance.

Additional records of the non-ammonite fauna can be found in Richardson (1928–1930) and, particularly, in Bomford (1948).

Interpretation

The ammonite fauna enables recognition of the Aalenian Scissum, Murchisonae, Bradfordensis and Concavum zones, with most of their component subzones, the Lower Bajocian Discites and Sauzei zones, the Upper Bajocian Parkinsoni Zone and, probably, the Garantiana Zone, and the Lower Bathonian Zigzag Zone, with two of its subzones, as shown in (Figure 2.18). The Scissum Zone rests non-sequentially on the Bridport Sand Formation (?Lower Jurassic, Upper Toarcian). Non-sequences higher in the succession cut out the Ovalis, Laeviuscula, Humphriesianum and Subfurcatum zones.

Although the Aalenian Stage totals only 2.4 m in thickness at Horn Park Quarry, 14 of its 16 known ammonite biohorizons can be recognized; only Aa-1 (Leioceras opalinum) and Aa-6 (Ludwigia patellaria) are missing (Figure 2.18). Details of the diagnostic ammonite taxa of the Aalenian and Lower Bajocian biohorizons were given by Callomon and Chandler (1990) who also figured several specimens from Horn Park Quarry. The Aalenian ammonite biohorizons are all based on the single family Graphoceratidae (see (Figure 1.4), Chapter 1), representatives of which from Horn Park Quarry have been discussed and figured by Chandler (1996). The oldest Aalenian stratum is the Scissum Bed, a name first introduced to the Dorset succession by Richardson (1928–1930) (see also Burton Cliff and Cliff Hill Road Section GCR site report, this volume). Richardson (1928–1930) was also responsible for naming the Ancolioceras Bed (Bed 3a) after a graphoceratid ammonite genus that was considered to be a junior synonym of Leioceras by Donovan et al. (1981) and taken as one of that genus' macroconch subgenera by Chandler (1996). Bed 3b provided the first British record of the grapho-ceratid genus Staufenia, allowing a useful correlation with sequences in continental Europe (Chandler, 1982). The Craterospongia Bed (Bed 3c) was first named by Parsons (1980a) after a sponge genus for which Horn Park Quarry is the type locality (Dighton Thomas, 1948).

The richly fossiliferous Horn Park Ironshot Bed (Bed 5) (Figure 2.19) is the lateral equivalent of the Bradford Abbas Fossil Bed (see Bradford Abbas Railway Cutting GCR site report, this volume) but the Gigantea Subzone of the Aalenian Bradfordensis Zone (represented by the lower part of the Horn Park Ironshot Bed) is missing at Bradford Abbas Railway Cutting, and the younger part of the Bajocian Discites Zone (represented by the upper part of the Bradford Abbas Fossil Bed) is missing at Horn Park Quarry (Callomon and Cope, 1995); indeed, the latter zone is present only at the western end of the quarry where Bed 5e is thickest. Nevertheless, Callomon and Chandler (1990) considered that the section at Horn Park Quarry would do well as an English reference section for the Aalenian–Bajocian stage boundary (see also Seavington St Mary Quarry GCR site report, this volume). This local 'coming and going' of individual beds, which is characteristic of the Aalenian–Bajocian succession in this region, as well as the other sedimentological features (see Seavington St Mary Quarry GCR site report, this volume), are almost certainly the result of tectonic activity. According to Callomon and Cope (1995), the seven successive ammonite faunas (biohorizons Aa-11-Aa-16 and Bj-1) recognized within the Horn Park Ironshot Bed have been resolved only because of the abundance of ammonite material collected; otherwise the morphological overlap between the successive assemblages of the dominant graphoceratids (Chandler, 1996) would be too great for them to be distinguished. Bed 5a is known locally as the 'Dinner Plate Bed' because of the concentration therein of perfectly preserved and gigantic (up to 0.5 m diameter) Brasilia gigantea macroconchs. Specimens of the sonniniid ammonite Euhoploceras from the top part of the Horn Park Ironshot Bed are featured in Sandoval and Chandler (2000).

The Lower Bajocian Substage is represented exclusively by the Red Bed (Bed 7) and the clay parting at its base (Bed 6). The name (often used in the plural) originates with Buckman (1910a) who described it on the coast at Burton Bradstock (see Burton Cliff and Cliff Hill Road Section GCR site report, this volume).

According to Callomon and Cope (1995), the Astarte Bed (Bed 8), at the base of the Upper Bajocian Substage, is one of the few tolerably constant beds of the Inferior Oolite Formation in this region — others being the Red Bed (see above) and the Scissum Bed (Bed 2). Named after an astartid bivalve, now referred to the genus Neocrassina, it has elsewhere locally yielded ammonites diagnostic of the Garantiana Zone (see Seavington St Mary Quarry, Bradford Abbas Railway Cutting, Louse Hill Quarry and Halfway House Cutting and Quarry GCR site reports, this volume). Its characteristic fauna includes belemnites, bivalves, brachiopods, echinoids and gastropods (e.g. Wilson et al., 1958; Senior et al., 1970).

The ammonite fauna of the overlying 'Burton Limestone', so named by Parsons (1975b) (see Burton Cliff and Cliff Hill Road Section GCR site report, this volume), is indicative of the terminal Bajocian Parkinsoni Zone. Although Callomon and Cope's (1995) illustration (fig. 10) indicated the presence of two ammonite bio-horizons within the Burton Limestone at Horn Park Quarry, as shown in (Figure 2.18), full specifications for these and others in the Upper Bajocian succession have not been published and are not yet fully resolved (cf. Callomon and Cope, 1995, fig. 7; and Callomon, 1995, fig. 3). Indeed, Callomon (1995) commented that the faunal succession of the Garantiana and Parkinsoni zones in general had so far received little more than cursory attention.

The Zigzag Bed, the youngest bed of the Inferior Oolite Formation at Horn Park Quarry, belongs to the Lower Bathonian Zigzag Zone (see also Burton Cliff and Cliff Hill Road Section GCR site report, this volume). The many ammonites in Bed 11 include almost the entire known fauna of the Zigzag Zone (Arkell, 1951a, 1958a; Torrens, 1974) and enable recognition of the Convergens Subzone in beds 11a and 11b, and the Macrescens Subzone in beds 11c and 11d (Callomon and Cope, 1995).

Conclusions

The succession at Horn Park Quarry possibly represents the most complete record of the Aalenian and lowest Lower Bajocian successions in southern England. The richness of the ammonite fauna, much of which is perfectly preserved, has made it a world famous and key locality for Middle Jurassic stratigraphy. It has played a major role in the elucidation of the regional stratigraphy, particularly of the Aalenian Stage, and has enabled the type material of many of the ammonite species first described by S.S. Buckman to be accurately pinpointed. It includes the richest development of the upper Aalenian Stage in Dorset. Horn Park Quarry is thus a locality of both national and international importance for Aalenian–Bajocian stratigraphy as well as being a prime palaeontological site.

References