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Ichnofabrics Containing Ophiomorpha: Significance in Shallow-Water Facies Interpretation

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Ichnofabrics Containing Ophiomorpha: Significance in Shallow-Water Facies Interpretation Journal of the Geological Society, London, Vol. 150, 1993, pp. 149-164, 9 figs. Printed in Northern Ireland Ichnofabrics containing Ophiomorpha: significance in shallow-water facies interpretation J.E. POLLARD ~,R.GOLDRING 2&S. G.BUCK 3 1Department of Geology, University of Manchester, Manchester M13 9PL, UK 2postgraduate Research Institute for Sedimentology, University of Reading, PO Box 227, Whiteknights, Reading RG6 2AB, UK 3Z & S Geology Ltd, Glover Pavillion, Campus 2, Aberdeen Science & Technology Park, Balgownie Road, Bridge of Don, Aberdeen AB22 8GW, UK Abstract: In an attempt to interpret Ophiomorpha ichnofabrics observed in core, three ichnofabrics are described from outcrops where O. nodosa is a conspicuous element. These ichnofabrics enable sandy shoreline sedimentary environments to be characterized and differentiated: (1) shoreface with mottled- Ophiomorpha-Planolites ichnofabric generally without primary lamination; (2) offshore tidal shelf sand wave facies with Macaronichnus-Ophiomorpha ichnofabric associated with primary, mainly cross- laminated or cross-bedded sands; (3) estuarine facies with Ophiomorpha ichnofabric associated with pri- mary lamination and, commonly, heterolithic sands and mudstones. Distinctions between the ichnofabrics are attributed to differences in primary stratification, the total ichnocoenoses, morphological features (such as burrow attitude, shaft restriction, pellet wall lining), to the nature of the substrate and, particu- larly, to the time available for colonization (larval settlement or relocation) and burrow construction, referred to here as the colonization window• The analysis is applied to an interval of core (Upper Jurassic, Central Graben, North Sea) and a sequence in Eocene sediments in southern England. Ophiomorpha nodosa Lundgren is one of the most widely 20mm quoted and widely known trace fossils. It is also a well-estab- lished and conspicuous trace fossil in shallow-marine sandy facies from the Mesozoic onwards. The principal account of its morphology and facies significance is by Frey et al. (1978). This pellet-lined burrow (Fig. 1) is today found over a range of~ nearshore environments, including lagoon and estuary floors, wherever the substrate consists mainly of sand grade sediment. Ophiomorpha is also known from ancient offshore deeper • .- .~., ,. water sediments where it is a not infrequent post-event trace in more proximal turbidites (e.g. Crimes 1977; Crimes et al. 1981). Several papers have recorded Ophiomorpha from non-marine facies. The occurrences reported from fluvial, deltaic and lacustrine Lower Cretaceous Wealden sediments of southern England (Kennedy & MacDougall 1969; Stewart 1978) appear not to refer to true Ophiomorpha (pellet-lined burrows) but to irregular burrows with a mudchip infill and bioglyph wall markings. However, Bown (1982) and Merrill (1984) claimed • • o " . the occurrence of Ophiomorpha in non-marine facies in the Cenozoic, and Chamberlain (1975) illustrated Ophiomorpha- \ like chimney-structures produced subaerially above burrows \ made by modern freshwater crayfish Asticus. \ The objectives of this paper are to review ichnofabrics with Fig. 1. General characteristics of (A) dissected Ophiomorpha in modern shallow marine depositional en- Ophiomorpha nodosa. burrow with sandy wall pellets, smooth inner surface to lining and vironments and to record some ancient ichnofabrics where muddy sand restriction; (B) taper of a restricted burrow (Bracknell); Ophiomorpha is common, so as to discriminate between (C) two shafts with taper into muddy layer, assumed to be several nearshore-shoreline facies, and show how these may colonization surface (Shellingford); (D) shaft truncation. Laterally, be identified and interpreted in core. Our material is some- lamination shows no indication of sedimentation break• Situation what limited in view of the very widespread occurrence of somewhat unusual because pelleted wall is usually more resistant to Ophiomorpha. Analysis is based on cores from the Upper Jur- erosion and sand of shaft less so (Bracknell); (E) base of restricted assic of the Central Graben (North Sea) and outcrops in the shaft (length 0.6m) leading into T-junction with another gallery Upper Jurassic, Lower Cretaceous, Palaeocene and Eocene of (Bracknell); (F) lined shaft passing down into humic rich sand southern England. Most emphasis is placed on Eocene without lining. Humic sand truncated and burrowed by Thalassinoides sections. cf. suevicus (Fig. 8A). Bar scales A, C, D, E 10mm. 149 Downloaded from http://pubs.geoscienceworld.org/jgs/article-pdf/150/1/149/4892172/gsjgs.150.1.0149.pdf by guest on 02 October 2021 150 J. E. POLLARD ET AL. Ophiomorpha in modern shallow water environments silty sand to silty mud. In shoreface sands the laminated sand is less strongly bioturbated by polychaetes. The inner offshore Despite the widespread occurrence of pelleted callianassid zone, where storm-derived silt is present, shows complete de- burrows in Recent sediments (Frey et al. 1978) only three stratification by Echinocardium, and contains burrows with a offshore to shoreface transitions and one study of subtropical thick silt lining (Palaeophycus heberti) (Reineck & Singh 1971, estuarine sediments have been described in modern environ- fig. 6). ments that allow some appreciation of the ichnofabrics In these profiles the ichnofabrics have not really been de- developed. scribed in sufficient detail to allow close comparisons to be made with the ancient ichnofabrics described below. The Georgia coast." low energy, tide dominated gulf-shoreface. The studies of modern sediments show that Ophiomorpha presents most complete description is from the gently sloping, tide- particular problems for ichnofabric analysis of ancient facies dominated low energy Georgia Coast (Howard & Reineck since in so many instances it was formed to a deep level and had 1972; Hertweck 1972) where the lower offshore zone (> 10 m) is a high potential residence time. In boxcores and in cores the in relict medium-to-coarse grained sands and only the problem is aggravated by the limited sample area. Bio- shoreward, upper offshore to shoreface sands are of present-day logically, the original burrow system related more closely to sedimentation. Storm-wave activity is low in the area and the environment that pertained at the original burrow connec- leaves no significant stratification. The distribution of biotur- tion to the sediment-water interface than to the immediately bation is variable across the shore-offshore transition but adjacent sediment (Frey & Goldring 1992). Where environ- exceeds 50% (generally 90-100%) in the upper offshore zones mental parameters change relatively rapidly, as is often the (2-10 m). Mud is present only in minor amounts. Where biotur- case in tidal nearshore environments, close association of the bation is greater than 90% reworking is so thorough that it ichnotaxa present does not necessarily imply original tiering of cannot be related to specific organisms and when between 50% an aggradational substrate. Hence it is rare that Ophiomorpha and 90%, individual structures are not obvious. In both cases and any associated traces can be integrated into a tier diagram reworking has been assumed to be due to animals found in the such as has been done for more uniform facies and event stratifi- boxcores. There is a diverse infauna but, 'overprinting' is often cation as described by Bromley & Ekdale (1986) and Ekdale & intense and Hertweck (1972) concluded that only three species, Bromley ( ! 991 ). Callianassa biformis (in the upper offshore zone, leaving a ramifying system of knobbly-lined burrows: Ophiomorpha) associated with Squilla (a mantid shrimp leaving a simple-lined Methods of ichnofabric analysis burrow) and Glyceris (a polychaete producing a mucus-lined Core material examined has been slabbed, and generally burrow) are responsible for the bioturbation that remains. lightly moistened, to display trace fossils more clearly. All outcrops examined are in soft, loose, fine- to coarse-grained Georgia coast." mesotidal riverine and salt marsh estuaries. These sand or coarse silt with some muddy intervals. Most are at were studied by Howard & Frey (1975, 1980a,h), Frey & steep to vertical sea cliffs or quarry faces. Some latex peels Howard (1980) and Mayou & Howard (1975). It is the strong have been taken but it has been found that the best contrast is lack of lateral continuity that characterizes the estuary facies in obtained by scraping moist, but not wet, sediment (in spring or respect of grain size, sedimentary structures and bioturbation, early summer) and then photographing. Systematic scraping and contrasts with the greater lateral persistence of shelf sedi- and sketching was used to determine three-dimensional form. ments of the same area. This reflects particularly the rapid This was not always successful owing to the nature of the out- changes in hydraulic regime. The point bars exhibit an unusual crops and because of traces crossing and loss of burrow con- assemblage of traces that are normally found over a range of tinuity. At the same time sketches of the ichnofabrics were depositional environments. Bioturbation is most intense in the made, generally at xl scale, and in many cases the fabrics were middle reaches of estuaries. Howard & Frey (1980a) em- quantified using the methods explained in Taylor & Goldring phasized that it is not simply the primary structures, dominated (1993). The graphical method is useful in distinguishing
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