Journal of the Geological Society, London, Vol. 150, 1993, pp. 149-164, 9 figs. Printed in Northern Ireland

Ichnofabrics containing : significance in shallow-water facies interpretation

J.E. POLLARD ~,R.GOLDRING 2&S. G.BUCK 3 1Department of , 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 wave facies with Macaronichnus-Ophiomorpha ichnofabric associated with primary, mainly cross- laminated or cross-bedded ; (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 , Central Graben, North Sea) and a sequence in in southern .

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 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 . 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 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

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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 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, 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 the by heterolithic stratification, especially flaser and lenticular several ichnofabrics. We believe that it can be applied mean- bedding, that are diagnostic of the estuaries but the combina- -ingfully to cores as representative of a well defined zone of tion of the physical and biological features. ichnofabric several metres in thickness, though not just isolated core samples (see applications below and Taylor & California coast: high energy shelf-shoreface. Howard & Rei- Goldring 1993, figs 1, 4). neck (1981) described the facies of the high energy beach-to- The photographic illustrations have been selected to relate offshore sequence off the California coast. Storm-generated to slabbed core as well as to morphological detail observed on units are typical of the offshore and transition zones (equiva- weathered outcrop faces. Quantitative assessments are based on lent to the lower shorefaces of Sapelo Island). Bioturbation by areas considerably greater than those illustrated. Dendraster (echinoid) is prominent in the lower shoreface but does not obscure primary lamination and leads to an irregular coarse festoon-like fabric (Howard & Reineck 1981, fig. 9c). lchnofabrics containing Ophiomorpha Bioturbation of the storm-generated units has not been fully Ichnofabric 1." Mottled sand-Ophiomorpha-Planolites described but was assumed to be the work of polychaetes. (Figs 2,3) Gulf of Gaeta (Italy): non-tidal, low energy, steep, shoreface to This ichnofabric is based on the Becton Sand Formation offshore transition. The shoreface to offshore transition is at (Eocene) (Edwards & Freshney 1986; Bristow et al. 1991) at the about 6 m depth (Reineck & Singh 1971). In this zone the sedi- Barton on Sea, coastal section where the lower ment is silty-sand with mud increasing with depth. The main member is almost wholly of this ichnofabric. The Becton Sand bioturbator, Eehinocardium cordatum, completely reworks the follows by transition (Fig. 2C) the muddy Chama Sand with

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A 1 10 100 I t J .f

80

,.v O. nodosa 6

t2 PI. montanus 10.7 ~. PI. beverleyensis

1.2 ] \ ~. ~Palaeophycus isp. cm 70 onoos IC

1 10 100 I I !

-Io.~ ~

1.85 I reichichnus isp.

echinoid ",x ,' ~,, I~l~

__J 0.2 Cylindrichnus --~ 0.2 ==~\PI. montanus (sand) 0.5 . % ¢ Pl. montanus (mud} 0.6 l O" n°d°sa 0

1.0 Thalassinoides , /~.

Fig. 2. Ophiomorpha-Planolites Ichnofabrics and constituent diagrams (BI 5-6). Becton Sand, Barton on Sea (SZ 249926). (A) typical fabric truncated by erosion surface to Becton Bunny Member with truncated, large diameter Ophiomorpha (probably of omission suite) with thick and muddy wall. (B) typical ichnofabric, approx. 1.0 m below erosion surface to Becton Bunny Member. Constituent diagram similar to above but without second O. nodosa. (C) transition Ichnofabric l b with sparse Ophiomorpha.but with spreiten burrows. Scale bar approximately 6.0 m below erosion surface. Scale bar in A, B 50 mm. In the consituent diagrams the horizontal axis corresponds to percentage cover by primary sediment fabric (if present)and each burrow type. Vertical axis with type of primary sedimentation, followed by each ichnotaxon in order of burrowing (from cross-cutting relationships), and shown by cross-section dimension. Wavy line indicates hiatus; BI, bioturbation index (Taylor & Goldring 1993).

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shafts (below) absent to rare. Turn-arounds in shaft (Frey et al. 1978, fig. 11) not observed. The three-dimensional form of the burrows is unknown but Y-junctions are typically expanded. P. montanus is conspicuous as dark, muddy-sand burrows winding through the sediment at all trends to stratification. Generally more or less evenly distributed, but occasionally localized within, through and around Ophiomorpha. Crossings are apparent but no branching observed. Other traces include Palaeophycus, , Taenidium, Terebellina (rare).

Variation. l a, occasional areas where Ophiomorpha may be nearly absent so that the ichnofabric is mottled with scattered P. montanus (Fig. 3); l b, mottled with spreiten burrows and occasional Ophiomorpha (Fig. 2C).

Fig. 3. Ophiomorpha Planolites Ichnofabric l a with sparse to absent Occurrence. Upper Jurassic, Fulmar Formation, Central Ophiomorpha, Becton Sand, Barton on Sea, approx. 3.0 m below North Sea Basin (above and below); Eocene, Becton Sand erosion surface to Becton Bunny Member. Scale bar 50mm. Formation, Barton on Sea (Bristow et al. 1991, with earlier references).

loss of and silt except for traces of former muddy partings Environmental significance and discus.s'ion. This ichnofabric dis- disrupted by bioturbation. At about 6.5 m below the top of the plays sharp contrast between the indistinct mottling and the member and prior to the appearance of Ophiomorpha galleries distinct ichnotaxa. The disparity is due, either to repeated omis- (at 5.8 m below top) the sediment is almost completely biotur- sion overlap by two discrete trace fossil suites, or to complex bated with mottlings: Planolites montanus, Palaeophycus tiering with the gradual aggradation of a tiered suite. Knowl- heherti and Asterosoma isp. ( Cylindrichnus concentricus of Ffir- edge from modern situations (above) is insufficient to use as sich 1974), spreiten burrows (possibly due to irregular ec- analogues. In either model aggradation was probably by small hinoids) but also retrusive spreiten attributable to Teichichnus (cm) increments because lined and unlined burrows are closely isp. and resembling the ichnofabric of the inner offshore zone of associated. The absence of primary lamination in a sand- the Gulf of Gaeta (above). Scattered valves of Chama are dominated lithology points to relatively slow aggradation present, though local pockets of the dissociated, nested to under moderately high hydraulic conditions. But the uniform irregularly distributed valves indicate infil!ing of large (30- dispersion of Ophiomorpha galleries suggests that the amount 40ram diameter) burrows (cf. Thalassinoides). The lower of burrow overprinting was low. The Becton Sand is devoid of member of the Becton Sand shows little change in ichnofabric shelly fossils and was probably decalcified at an early stage of (Fig. 2B) over the 6 m thickness. The member is truncated by an burial. Marine shells would be expected in association with almost planar surface with scattered pebbles and cobbles this ichnofabric. Lower to middle shoreface. (Plint 1988a, b; top of cycle 2 of Hooker 1986). Just below this erosion surface the ichnofabric is cut by galleries of Ophiomor- Comparisons. This ichnofabric has not been widely cited but O. pha larger than below and with muddier pellets and which are nodosa has been associated with storm beds, commonly with also truncated by the erosion surface (Fig. 2A). It is also a hummocky cross-stratification (e.g. Frey 1990; Frey & Howard common ichnofabric in cores from the Upper Jurassic, Fulmar 1990; MacEachern & Pemberton 1992), where the burrows were Formation (Humber Group) of the Central North Sea Basin initiated from the top of the storm bed or from a higher horizon (UKS Quad 30) where in upward shallowing successions (para- (Frey & Goldring 1992). Vossler & Pemberton's (1989) assem- sequences) it follows Anconichnus ichnofabrics (Goldring et al. blage 5 of the Cardium Formation (Upper Cretaceous, 1991), though frequently there seems to be an interval, of vari- Alberta) seems to refer to this ichnofabric as does thin-bedded able thickness, between the loss of Anconichnus and the appear- sand and mud lithofacies (transition zone or lower shoreface) ance of Ophiomorpha. of Mt Laurel Formation (Upper Cretaceous, New Jersey; Mar- tino & Curran 1990). Diagnosis. Primary lamination absent or rare (Bioturbation The main transect from Sapelo Island, Georgia (above and Index, BI 5-6: Taylor & Goldring 1993). Background of in- Hertweck 1972, pl. 1-3) suggests strong similarity between the tense, but patchy, ovoid, indistinct mottles cut by distinct O. offshore (12m depth) echinoderm-bioturbated sand and near- nodosa and P. montanus. to-shoreface burrowing by Callianassa biformis.

Description. Fine- to very fine-grained sand. Primary lami- Ichnofabric 2." Macaronichnus-Ophiomorpha nation or mud partings rarely present. Soft sediment deforma- tion uncommon to absent. Background of intense but indistinct ichnofabric associated with primary lamination (Fig. 4) mottling of mostly incomplete ovoid patches which cannot be This ichnofabric is based on observations of sections in the traced into the sediment to any significant distance. O. nodosa Woburn Sands (Lower Greensand, Aptian, Lower Cretaceous) typically evenly distributed (Fig. 2B), with galleries more of the Leighton Buzzard area, southern England (Buck 1987), prominent than shafts. Shafts lined or unlined and galleries though it was figured first by Middlemiss (1962). lined, unlined or roof-lined with occasional spreiten laminae (cf. Kennedy & Sellwood 1970, Fig. 2E) or laminated fill. Diagnosis. Cross-stratified (occasionally parallel laminated), Wall of muddy sand with pellets generally well formed. fine- to coarse-grained sand with often dense Macaronichnus cut Burrow diameter (external) between 10 and 35 ram. Restricted by Ophiomorpha.

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Fig. 4. (A-C) Ophiornorpha-Macaronichnus Ichnofabrics (BI 3-4), Woburn Sands, Lower Cretaceous. (A and B) Macaronichnus Ichnofabric 2b: large-scale cross-lamination with M. segregatis (arrowed) associated with occasional chevron structure (?water escape), Pratt's Quarry, Leighton Buzzard (SP 930241); (C)cm~lm parallel and cross-laminated sand with Macaronichnus (Ma) crosscut by O. nodosa, Skolithos isp. and Planolites isp. in Ichnofabric 2a, Chamberlain Barn Quarry, Leighton Buzzard (SP 927265); (D) probably referable to Ichnofabric 3c, primary lamination cut by O. nodosa (On), ramified by Planolites (PI) and very fine ?Skolithos isp. (Sk) Chamberlain Barn Quarry, Leighton Buzzard. Scale bar for C, D 50 mm.

Description. Typically this ichnofabric is associated with shafts and galleries, with well-formed lining but sparse dis- centimetre, decimetre or metre thick beds of cross-laminated tribution. At certain horizons between the Macaronichnus lay- sand or thinly interbedded muddy sand and clean sand with an ers dense mazes of Ophiomorpha may be developed in fine- to erosional base. Within these beds, zones up to 0.2m thick are medium-grained, sometimes pebbly sand. Here Ophiomorpha is intensely bioturbated by M. segregatis (Clifton & Thompson often enwrapped or associated with intertwining masses of 1978) about 10mm diameter with pale sand infill and thin sinuous, vertical to horizontal, fine, cemented burrow fills darker wall lining. Burrowing is predominantly parallel to 1-2mm diameter, probably referable to Planolites and the planar lamination and concentrated in the lower parts of Skolithos. Shafts 10mm diameter with cemented but unpel- the thicker-bedded units but oblique to cross-lamination, al- leted walls, Skolithos isp., cross-cuts both the Ophiomorpha though more generally distributed throughout the heterolithic and the Macaronichnus layer. It may represent weakly lined intervals. Overall the sediment shows a fabric of pale 'mac- shafts of Ophiomorpha or an infaunal suspension feeder. aroni-like' burrow fills. Macaronichnus does not weather out in relief in contrast to other traces. Variation. 2a, Discrete Macaronichnus zones cross-cut by Ophiomorpha cross-cuts the Macaronichnus fabric, both Ophiomorpha, Skolithos, +_ Planolites (Fig. 4C); 2b, Macaroni-

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chnus zones without Ophiomorpha, +_ ?Conichnus or chevron estuarine point-bar sedimentation. (1) locally at Shellingford collapse structures (Fig. 4A, B), Macaronichnus oblique to Quarry and Sandford Quarry in the Highworth Grit (Upper bedding. Jurassic) of the Faringdon area, Oxfordshire (Ichnofabric 3b). Outcrops in the area were briefly described by Arkell (1947, Occurrence. 2a, Leighton Buzzard area, Woburn Sands, (Stone with earlier references). Neither quarry has been described sedi- Lane Quarry, Pratt's Pit); Boscombe Sand, Hengistbury Head mentologically but the Highworth Grit has formed sections of (Fig.8A); Fulmar Formation, Central North Sea (isolated unpublished theses by Talbot (1973a), Ali (1978) and Harvey zones up to several metres thickness). 2b, Leighton Buzzard, (1986). The overlying limestones of the Osmington Oolite Woburn Sands, Pratt's Pit, in cross-beds up to 4 m thick. (The Formation were described by Ali (1984) and Talbot (1973b); absence of Ophiomorpha is attributed to the much greater (2) an undescribed landfill site at Chavey Down, Bracknell, thickness of the beds.) , in the uppermost Bagshot Formation (Eocene) (Ich- nofabric 3a); (3) two sections in the Palaeocene/Eocene des- Environmental significance and discussion. The scale of cribed by Kennedy & Sellwood (1970) at Knowl Hill, bedforms and large-scale compound cross-bedding of the Berkshire (Ichnofabric 3c) and at Upnor, Kent (Ichnofabric Woburn Sands Formation supports their general interpreta- 3b). The stratigraphical position of the sand unit with tion as large-scale subtidal sand waves formed in a tidal sea- Ophiomorpha at Knowl Hill is uncertain. It may be considered way (Buck 1987; Bridges 1982). The sands extensively biotur- either as the top of the Reading Formation (Kennedy & bated by Ophiomorpha-Macaronichnus-Planolites-Skolithos Sellwood 1970) or the (basal) Twyford Member of the London are regarded as formed in a more offshore situation than unbur- Clay (King 1981; Sellwood 1974). Channel-fill sediments in rowed sands with a variety of high energy tidal features in- the Woburn Sands (Chamberlain Barn Pit) (Fig. 4D) are prob- cluding mud-draped foreset-bundles, herring-bone cross-bed- ably referrable to Ichnofabric 3c. ding and slumped beds. Macaronichnus is believed to result from extensive deposit feeding of worms virtually immediately Diagnosis. Primary lamination with Ophiomorpha; Ichnofabric after deposition, perhaps up to 10 cm below the sediment-water 3a, with low frequency of well structured Ophiomorpha cutting interface (Curran 1985; Saunders & Pemberton 1990). The Conichnus and associated with parallel lamination, cross- cross-cutting Ophiomorpha with the associated filamentous lamination and + inclined heterolithic stratification; Ich- Planolites-Skolithos burrows were probably initiated from a no['abric 3b, with low frequency of well-structured Ophiomor- higher surface during a (short) depositional hiatus. The zones pha associated with parallel lamination, cross-lamination and of more abundant shafts and galleries of Ophiomorpha often + inclined heterolithic stratification; lchnofabric 3c dense and associated with coarse or pebbly sand perhaps suggest higher variably structured Ophiomorpha associated with parallel lam- energy conditions analogous to the lower foreshore, upper ination and cross-lamination, occasional muddy laminae/ shoreface transition zone. The variations in this ichnofabric heterolithic stratification. therefore are believed to reflect the time available to colonize Other traces are irregularly and sparsely distributed except the shifting sands rather than major large-scale changes of for Skolithos which is common in thin event beds in Ichnofabric depth or sedimentary environment (below). 3a. P.montanus is generally absent in Ichnofabric 3a but locally present in 3c. Other traces are given below. Comparisons. These respective trace fossil associations and ichnofabrics are virtually identical to those described by Cur- Description. Ichnofabric 3a. Typically the lithofacies com- ran (1985) from the Lower Cretaceous Englishtown Formation prises cm~lm parallel laminated fine-grained sand alternating of the Atlantic Coastal Plain in Delaware, USA. Curran withcm units of inclined heterolithic stratification sand with attributed Macaronichnus cross-cut by sparse Ophiomorpha and small-scale cross-lamination and mud drapes, cut and fill Skolithos to lower shoreface attached sand bars, and dense structures and frequent intraformational conglomerate. Soft Ophiomorpha boxworks with enwrapping filamentous burrows sediment deformation (water escape) are common (cf. Gold- to lower foreshore/upper shoreface situations. Other studies of ring 1991, Fig. 5.1): typically originating at a large Ophiomor- Macaronichnus ichnofabrics in cores of Cretaceous pha gallery where a roof pellet has failed. Ophiomorpha dis- in western Canada assign them to upper shoreface/foreshore or persed, well lined shafts and galleries (Fig. 5A), occasional rarely, intertidal environments (Rahmani & Smith 1988; roof-lined lengths. Y-branching with expanded junctions Ranger et al. 1988; Saunders & Pemberton 1990; MacEachern forming U-structures occur with W-structures and, possibly, & Pemberton 1992). Very rare examples of these ichnofabrics simple mazes, but three-dimensional pattern is unclear. (2a Tilje Formation, central Norwegian Shelf, 2b Etive Boxworks are unproven. Shafts and galleries show consider- Formation, Brent Group) are recorded by Taylor (1991) from able range in size between beds, within a bed and between North Sea cores although they have not been clearly recog- linked shaft and gallery. The latter is most emphasized when nized in the Fulmar Formation in this study. The ich- shafts reach 25-30mm diameter, where the connected gallery nocoenoses of Ophiomorpha, vermiform burrows (Macaroni- may be up to 55mm. Shaft restriction common (below) but chnus?) and chevron collapse structures recorded by Asgaard & found only in association with such disparity of size. Shafts are Bromley (1974) from Miocene littoral sands of Bornholm ap- typically normal to and galleries parallel to stratification. pear close to Ichnofabric 2a. Preserved tops to shafts occasionally flare to a cone with unlined wall or tapering within a unit of heterolithic sediment (Fig. 1B). Ichnofabric 3: Ophiomorpha with primary lamination (Fig. 5) Conichnus discrete, occasionally grouped, retrusive adjust- ment structures with circular outline often leading upwards to This ichnofabric is principally based on four main sections a cylindrical sand-filled tube. Base smooth, hemispherical to interpreted as estuarine sedimentation, two with inclined conical. Height of adjustment is variable due to truncation. heterolithic stratification (Thomas et al. 1987), representing Maximum height observed 70mm. Other burrows include

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A 1 10 100 C 1 10 100 I I ,1 I I A -1 Conichnus --~.: _%lz -~

O. nodosa

,~s~., ~7 ~ a ¢~ PI. montanus "~ "-"

t cm f

Fig. 5. Ophiomorpha Ichnofabric 3 (BI 1-2) with constituent diagrams. (A) Ichnofabric 3a, parallel and cross-laminated sand with Conichnus (adjustment: equilibrichnion) cut by Ophiomorpha (no example of cross-cutting in figure), Bagshot Formation, Eocene, Chavey Down Quarry, Bracknell (SU 896691); (B) Ichnofabric 3b small J form of Ophiomorpha originating at muddy lamina, cutting primary lamination, Highworth Grit, Upper Jurassic, Stanford Quarry (SU 327943); (C) Ichnofabric 3c, Ophiomorpha, lined and unlined shafts and galleries with upward density increase cutting primary lamination. Occasional horizons (not in figure) with Planolites montanus and Conichnus isp. Reading Formation/, Palaeocene/Eocene, Knowl Hill (SU 319798). Scale bar in C 50mm.

Taenidium isp., Asterosoma isp., Palaeophycus tubularis, Tri- Comparisons. Bockelie (1991, p. 211, Fig. 3B) described three chichnus (all uncommon), Arenicolites (rare). Skolithos rami- types of Ophiomorpha ichnofabric from Jurassic sediments of fies local cm thick sand units interpreted as event beds. These the Norwegian North Sea, each with low burrow density occurrences lack Conichnus but are cross-cut by Ophiomorpha. associated with primary stratification in medium to coarse- Relationships with other other facies and ichnofabrics are un- grained sandstones. A shallow-water environment was sug- certain. gested and we would suggest that an estuarine rather than open marine or shoreface situation is indicated. Similarly, the fabric Environmental significance and discussion. The presence of figured by Bromley (1990, Fig. 11.3) with sparse O. nodosa Ophiomorpha, generally with even distribution, in association cutting Skolithos isp. suggests a high energy estuarine bar en- with primary sedimentary features of inclined heterolithic vironment. The planar-laminated sands with burrows of stratification associated with point bar sedimentation, Callianassa major figured by Howard & Frey (1975, Fig 43; strongly suggests deposition and colonization of an estuarine 1985, Fig. 9A) from Georgia estuaries are also similar though a point bar. specific setting was not given. The facies and ichnofabrics agree In core expression this ichnofabric may be difficult to recog- in general with estuarine facies described from the Eocene by nize because of the low chance of the core intersecting Bosence (1973) and Goldring et al. (1978), but both these sites Conichnus, but the heterolithic stratification and stratigraph- (now infilled) were situated in the floor of a tidal channel ical variation in burrow size would be useful indications. subject to intense penecontemporaneous erosion. In contrast to

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A bedform advance .. continuous colonization 20 - 70 m/yr treadmill COLONIZATION WIND 0 W 3.5- 12 months

20 m abandoned burrows p.r 60 m deposition

erosion

I "~'~, , \' '.~ " " g'/"" °o, o'n,z., ,on I .'~ .:'X-:',:;." "-I ,'/.' Y.-,;-4.,.continuous Fig. 6. Diagram to illustrate ~- i-~!!~,~.~ ~"~ I: ilnd°w ~...-/~.~jT,/:. • ~3. ,~. colon izat io n colonization window. (A) sand wave migration and opportunity for . m-, ...;.~" ~ Y'-/~.~.,..-.~ ~ ' tJ window colonization of the trough area (Buck "_----<-;~-~'--'-;~'i\ ~-'--~~: spring tide __ __..,0~,... ' ,..._~ deposition -~-.__ /.'. ".~ .~. 1987); (B) schematic diagram to ,j .t~ ~" , , , , , ''o illustrate depositional processes on estuarine point bar where opportunity ! . . ~ .,',, .? .L " "'~ ,.,, ..- . ,.-. fo for colonization was limited to periods O~ neap tide of mud deposition; (C) shoreface ~, 1 deposition -.: ~ :,' "'~,< (t':, t.-. environment where physical restraints on colonization probably were limited to infrequent storms.

Bracknell, Arenicolites isp. (with one limb typically showing Comparisons. Howard &Frey (1975, pl. 9, Figs. 42, 43; 1980a, outward migration) was prolific at the top of 'event' sand units. Fig. 6.12a) showed similar fabrics from a Georgia estuary.

lchnofabric 3b. Fine-grained parallel-laminated or small- Ichnofabric 3c. Ophiomorpha dense and variably structured scale cross-laminated sand or cross-lamination in centimetre- with well-pelleted galleries cut by unlined galleries decimetre units alternating with cm units of heterolithic mud/ (Fig. 5C). Shafts sometimes indistinct. Associated with fine- sand (+ inclined heterolithic stratification) as in Ichnofabric grained, parallel-laminated sand and small-scale cross- 3a. Cut and fill, often with intraformational mud flake con- lamination, occasional mud intervals are of low lateral persis- glomerate common. Ophiomorpha well structured, generally tence due to penecontemporaneous erosion. Burrow density is narrower (5-12mm diameter) galleries and more steeply in- variable but progressive increase in frequency upwards from clined shafts than in Ichnofabric 3a, often J-form, but not unbioturbated base of unit, or following intra-unit scour. Oc- forming mazes (Fig. 5B). Shafts originate at mud drapes (fol- casional instances of Planolites montanus and/or Conichnus lowing small-scale cross-lamination) where the burrow is occur at specific horizons. Soft sediment deformation is gen- unlined and constricted (2-3 mm). Other burrows uncommon: erally absent. Planolites in mud. Soft sediment deformation is uncommon. (Diplocraterion parallelum is locally present but originates Relationships with other ichnofabrics. Gradual increase in abun- from overlying facies.) Stratigraphical relationships of the ich- dance of Ophiomorpha from unbioturbated sand occurs. Top is nofabric are uncertain. typically truncated. . En vironmen tal interpretation and discussion. The lithofacies gen- Environmental significance and discussion. This ichnofabric was erally associated with this ichnofabric is similar to that associ- figured by Kennedy & Sellwood (1970) and interpreted as rep- ated with Ichnofabric 3a but the ichnofabric is distinct. Absence resenting a littoral environment following transgression of of Conichnus and the generally small size of Ophiomorpha sug- Reading Formation paleosols. Extension of the quarry shows a gest that the depositional environment was located relatively more complex situation with: (1) a number of colonizations by further from an estuary mouth. In cores it might be difficult to the Ophiomorpha-forming associated with repeated cut separate this ichnofabric from Ichnofabric 3a. and fill, (2) extensive lateral facies change, with sands with

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Ophiomorpha cut out laterally by non-colonized sand-filled of the Woolwich Formation at Upnor as noted by Kennedy & channels, and (3) locally, a further unit of paleosol below the Sellwood (1970), and associated with the transgression surface transgressive base to the London Clay. A complex channel fill at the top of the lower member of the Becton Sand at Barton. is indicated with extensive cut and fill and repeated coloniz- Smaller shafts are associated with Ichnofabric 3b. ation. Local Conichnus and extensive penecontemporaneous Frey et al. (1978) described the variation across a seaward erosion suggests an estuarine (channel floor) rather than litto- profile with increasing density of burrow aperture and decrease ral or sublittoral 'depositional environment. Although first in depth of the maze or boxwork outwards from the shore where impressions suggest that Ophiomorpha forms boxworks, this is shafts are less prominent. The sections described do not show probably misleading, because of appreciable cross-cutting of this relatively simple relationship though galleries pre- galleries and the lateral variability of lined and unlined gal- dominate in Ichnofabrics 1. leries. Many shafts were apparently unlined, and where fill and matrix are of identical composition, the shafts can be infer- Time available for colonization by Ophiomorpha: the coloniz- red only where a structure passes into a diagenetically ation window. It is generally understood from the literature enhanced section. that the Ophiomorpha-animal colonized a sandy environment In core, this ichnofabric should be conspicuous though lat- such as is typically found on sandy lower beaches today and erally discontinuous. Gradual upward increase in gallery where it may be so conspicuous (Frey et al. 1978). But this abundance, and variability in wall lining are characteristic. association has then been extended to equating Ophiomorpha with a high energy depositional environment and linking it Comparisons. O. nodosa in the Fox Hills (U. Cret- with Skolithos (as in the Skolithos ichnoguild and Skolithos aceous) of the Denver Basin (Weimer & Hoyt 1964) resembles ichnofacies; e.g. Bromley 1990, p. 215, 217). However, analysis this ichnofabric in density and morphology of the burrows. The of the ichnofabrics described above suggest that the Ophiomor- section in the Pleistocene of southern Italy (D'Alessandro & pha-animal colonized muddy substrates during quieter con- Bromley 1986, pl. 9-14) suggests an estuarine depositional en- ditions in situations where change in hydraulic regime was vari- vironment, though the authors did not find evidence of tidal able in intensity and in periodicity. The several contexts of influence. They separated unlined burrows as Thalassinoides occurrence of Ophiomorpha point to colonization at a low suevicus (below). energy stage of a fluctuating sedimentary regime (Buck 1987; Harvey 1986). Buck determined the duration of the period available for colonization (which might be termed the Discussion Colonization Window) in open shelf sand wave facies of the The distinctions between the three ichnofabrics described and Lower Greensand (Fig. 6A; Ichnofabric 2). This is thought to their varieties can be attributed to four main factors. be between 3.5 and 12 months for the migration of large dunes with wavelength of 60 m, allowing for the formation of exten- The primary stratification. In Ichnofabrics 2 and 3 the primary sively and highly bioturbated intervals in the interdune region. stratification and the mottling of Ichnofabric 1 are, in many Colonization of the foresets by the Macaronichnus producer respects, sufficiently distinctive for satisfactory environmental was almost immediate. The sporadic occurrences of Ophiomor- interpretation. But problems arise, emphasized in core, where pha cutting Macaronichnus associated with the cross-stratifica- stratification is uncertain, or where it is uncertain whether tion record later colonization (a distinct suite) from the lower parallel lamination is related to upper flow regime in a chan- part of the lee slope of the dune or sand wave. In the estuarine nel setting or to upper flow regime under beach processes. environment envisaged for Ichnofabric 3a (Fig. 6B), the colonization window afforded by slack water at neap tides was Trace fossil assemblages. The differences between the trace short and probably of only hours to a few days duration. This fossil assemblages in the three ichnofabrics are relatively resulted in the relatively sparse distribution of traces in such small, generally insufficient to distinguish between them. facies and the relative incompleteness of the burrow systems Conichnus is probably the most environmentally significant present. At Bracknell the larger size galleries and restricted trace and its presence suggests normal marine salinity. But shafts possibly represent adult relocation rather than larval colonization by the Conichnus-animal seems to have been settlement (below), although size may also be related to prox- largely determined by hydraulic conditions; relatively slowly imity to the ecological limits of the taxa. Ichnofabric 3c results aggrading planar lamination (below). The minor ichnotaxa are mainly from repeated scour and fill leading to condensation unlikely to be sampled in cored intervals and are mostly of and burrow concentration rather than from nondeposition in facies-crossing ichnotaxa. However, Arenicolites isp. (rare in the inter-sandwave situation of Ichnofabric 2. Ichnofabric 1 Ichnofabric 3a at Bracknell) was prominent in the complex results from altogether slower sedimentation in a shoreface estuarine sediments of approximately the same age elsewhere environment with a more or less continuous colonization win- in the region (Bosence 1973; Goldring et al. 1978). dow (Fig. 6C). Morphology of Ophiomorpha. Several morphological The effects of penecontemporaneous erosion and minor features of Ophiomorpha and other traces seem to be environ- aggradation and degradation of the substrate surface have been mentally specific. These are described in the Appendix and much discussed in ichnological literature (see Bromley 1990; include: (1) shaft restriction, which is particularly common in Ekdale et al. 1984). These sedimentological factors are clearly the more distal (seaward) estuarine Ichnofabric 3a where it important at the sites described, but we consider that the time seems to be associated with large gallery size; (2) water escape available for larval settlement or adult relocation together structures which are common in Ichnofabric 3a, again associ- with the nature of the substrate to be more important factors in ated with galleries greater than 20 mm diameter; (3) the size characterizing the ichnofabric. Other, biological factors such range of Ophiomorpha galleries and shafts which is greatest in as periodicity of larva production, water temperature, nutrient Ichnofabric 3a, but large diameter shafts are also present at the availability and productivity would have been significant but base of Ichnofabric 1 at Barton, close to the transgressive base are indeterminable.

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1 10 100 1 10 100 1 10 100 I I J ! I I

! I c.80 ~- ~ ~ - --"~, .... i (1-2 cm) ";~ ~,~ ~jjl 80+ ,.'~'-~ ..,..~ -.,-~ ~'x. ~'x.( .. -~,.l 1 ~" "'~r.. _j'J c.15? c. 20 ~l cm)j'0\\ 1 i --c.1---..~ ,.~: IL.. J1 "~.-- Pl. montanus J :0.5?-- ~o Palaeophycu$ ? - ,:.1o? ,..~.~ cm ? ~ ,,~Terebellina

Fig. 7. Ophiomorpha ichnofabrics in cores of Jurassic sandstone from North Sea oilfields and constituent diagrams with approximate frequencies. (A) Fulmar Field, well 30/16-A30 (Ichnofabric 1); (B) Kittiwake Field well 21/18~, (Ichnofabric l); (C) quadrant 29, well 29/7-3Z (Ichnofabric 3b?). Scales in mm.

The trace fossil evidence also indicates that the periodicity of ichnofabrics (Johnson et al. 1986; Goldring et al. 1991). These the major heterolithic units was to be measured in hours to days Ophiomorpha sandstones can be compared to the ichnofabrics rather than seasonally as suggested by Thomas et al. (1987), thus described above. indicating rapid lateral migration of the point bar facies. The ichnofabrics in slabbed cores of the Fulmar Formation were measured over core lengths of respectively 1.3m (Fig. 7A) and 1.5m (Fig. 7B). The fine- to medium-grained Applications sandstones have a background bioturbation of mottled sand The value of this method of ichnofabric analysis is demon- patches cross-cut by oblique or horizontal Ophiomorpha with strated by application to two case histories. Firstly, palaeoen- thin, pelleted walls. Some horizontal Ophiomorpha possess vironments of Ophiomorpha ichnofabrics in Jurassic cores from only reinforced roof linings and rare layered floor deposits North Sea oilfields are analysed. Secondly, the controversial occur within galleries. Small dark-filled Planolites occur in depositional environments and sequence stratigraphic inter- both the matrix and the Ophiomorpha burrows infills. Wells pretation of the Boscombe Sand Formation (Eocene) at Hen- examined from the Central Graben (Fulmar and Clyde Fields) gistbury Head, are considered. frequently show greater diversity of associated trace fossils in the Ophiomorpha fabrics (including Palaeophycus and Terebel- lina (Fig. 7A) and rarely, ?Rosselia and spreiten burrows) than Interpretation of ichnofabrics of the Fulmar Formation in the thinner sequences of the more westerly Kittiwake Field. (Upper Jurassic), North Sea (Fig. 7) Here more intense bioturbation by thinner walled or collapsed The major reservoirs of several oilfields in the Central North Ophiomorpha burrows predominates (Fig. 7B). These Sea consist of moderately to completely bioturbated marine Ophiomorpha fabrics from the Fulmar Sandstones are broadly sandstones of Upper Jurassic age (Johnson & Stewart 1985; similar to Ichnofabric 1 from the Becton Formation (above) Brown 1986). The Fulmar Formation of the Fulmar, Clyde, and likewise are interpreted as having formed in a lower Kittiwake and Gannet Fields (Johnson & Stewart 1985; shoreface environment (compare Figs 2A, B and 7A). Johnson et al. 1986; Armstrong et al. 1987) is divided into An ichnofabric of low density (Bioturbation Index 2/3; several reservoir zones based on the nature of the sandstones Taylor & Goldring 1993) with discrete Ophiomorpha in a clean and their bioturbation (Johnson et al. 1986). The lower reser- cross-laminated fine-grained sand (Fig. 7C) associated with voir zones are dominated by muddy sandstones with varied erosion surfaces, has been studied in cores from an unnamed Anconichnus ichnofabrics (Goldring et al. 1991) which give way sandstone interval in the Humber Group of the Central upwards to cleaner sandstones with Ophiomorpha-Planolites Graben. This fabric is similar to Ichnofabric 3b above (corn-

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...... ,,,...... -.,.., .... .- ...... largely obscured 1991 ~~. _ . ~,.,~S~~~~l~ uoscomDe -:~ana .....--.-y.~'...... ~ • \ c 0 dewatering pipes 50m

vert. exaggeration x 3 E

gap 75m

Fig. 8. Generalized profile of Barton Group (Eocene) at Hengistbury Head, Dorset (SZ 171906) (after Plint 1983, 1988b) to show channel fill complex at top of Boscombe Sand and interval of pebble and cobble layers at base of Barton Clay. 1 and 2 indicate base of Barton Clay after Bristow et al. (1991) and Plint (1983, 1988b), respectively. (A) Irregular omission surface below pebble bed 2 with pre-omission Ophiomorpha (left) with humic-rich wall and burrow fill, eroded fragments of burrow and humic sand clasts (right). Thalassinoides cf. suevicus (Ts) cuts omission surface as does post-omission O. nodosa. Omission surface at this point is overlain by laminated sand with Macaronichnus isp.; (B) Post-pebble bed 2, laminated sand with rare Rosselia?(R), Palaeophycus and Macaronichnus isp. (Ma) cut by O. nodosa (On). (C) pre-channel sediment with mottled Ophiomorpha-Planolites ichnofabric. Pellet-walls are particularly rich in organic particles. Fabric is truncated by laminated (pale) sand below diagenetic boundary to humic-rich sand. (D) location of burrow in Fig. 1F. Scale bars 50 mm.

pare Figs 5 and 7C) and suggests formation in a higher energy of the channel fill, identified by Plint (1983) as type 2 kerogen, environment with a clean sand substrate perhaps upper and the liquefaction, fluidization and erosional structures de- shoreface or estuarine bar, particularly as it succeeds non- scribed by Plint (1983, 1988b), (see also Bristow et al. 1991 who marine coal-bearing strata (Planolites montanus ichnofabric) discuss previous research). The lower part of the Barton Group and brackish or marine influenced lagoonal sediments. (top of Boscombe Sand, and lower part of Barton Clay with These brief examples indicate the equally useful applica- Warren Hill Member) dip gently eastwards, the Boscombe tion to core interpretation of the type of ichnofabric analysis Sand passing below beach level rather more than halfway along developed here in outcrop situations. the section (Fig. 8). Plint (1988b) took the base of the Barton Clay some 1.5-2.0 m lower than Bristow et al. (1991), at the base of the first major cobble bed (Fig. 8, horizon 2). The difference in Boscombe Sand (Eocene) at Hengistbury Head, Dorset interpretation is explained (Bristow et al. 1991) as due to prac- (SZ 17 90) (Fig. 8) ticality in regional mapping. The interval between these two The much visited Eocene section on the south side of Hen- levels is occupied by laterally discontinuous sand facies and gistbury Head is of wide interest because of the humic-rich sand cobble layers and D. Curry (pers. comm. 1991) records moulds

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of marine bivalves from the sands. The unresolved questions Ichnofabric analysis adds to the interpretation of the Upper here are what was the sedimentary environment prior to the Boscombe Sands: (1) in confirming Plint's reading of the strati- incision of the channel, the extent to which the channel was graphy, though the multiple cobble beds at the top of the under marine influence, and what was the manner of transgres- Boscombe Sand may represent a more complex sequence of sion and relationship to sequence stratigraphy. events; (2) additional evidence on the development of the chan- The currently exposed sands below the channel fill exhibit nel; and (3) the suggestion that the pre-channel sediments may an ichnofabric close to Ichnofabric 1, with uncommon cross- represent shoreface sands, and that these sands link environ- lamination and mottled fine-grained sand with O. nodosa and mentally with the lower part of the Boscombe Sand. P. montanus. Ophiomorpha shafts are between 5 and 15 mm diameter, galleries are generally rather compressed, up to Conclusions (Fig.9) 40 mm diameter, often top-lined, with mud and carbonaceous pellets often deformed (Fig. 8C). Palaeophycus tubular&, Ophiomorpha is a useful, but not wholly reliable guide and Skolithos linear& and P. beverleyens& also occur. Hooker indicator of near-shore sedimentary environments in Mesozoic (1975) and Plint (1983, 1988b) regarded this facies as estuarine and Cenozoic strata. Taken in conjunction with the associated following transgression at the base of the Boscombe Sand but primary sedimentary structures, the ichnofabric (including the ichnofabric and continuity of the facies suggest a middle to associated ichnotaxa, burrow size and other morphological lower shoreface depositional environment. features), and the facies context, shoreface, shelf sand wave and The ichnofabrics of the channel fill are complex because estuarine environments can be distinguished with a high degree they represent superimposed: (1) colonization during success- of confidence, though more outcrops need to be investigated. ive stages in the cut and fill of the channel; (2) colonization Relatively little use can be made of modern studies on environ- associated with the truncation of the channel fill; and (3) ments where Ophiomorpha is being constructed. This is because colonization from above the truncation event. Pre-truncation of the paucity of information on modern environments that colonization is represented by relatively sparse shafts often have a high preservation potential in the fossil record. inclined to the regional stratification but which may reach a Carbonate-dominated facies also require investigation. Since length of 0.6m (Figs IF and 8A). The pellet-lined burrows are Ophiomorpha is essentially a trace produced by tropical and filled by humic-rich sand and represent a suite formed prior to subtropical these conclusions do not apply to both stages of soft sediment deformation (Plint 1983), and com- equivalent temperate environments and facies. pletion of the fill (below). Also, a few shafts in the local top of While Ophiomorpha is today constructed by a range of the mottled Ophiomorpha-Planolites ichnofabric are due to crustaceans under different ecological controls (and this must colonization from an earlier stage of channel cutting. Erosion have been the case in the past) it would seem that perhaps the of the pre-omission sediment led to an irregular surface most important control on Ophiomorpha ichnofabrics is the covered by occasional flint pebbles and cobbles, clasts of physical constraints of the colonization window. humic-rich sand, eroded fragments of Ophiomorpha and sections of Ophiomorpha wall standing proud (Fig. 8, horizon We are grateful to Shell Exploration and Production UK for the 2). The omission suite, Thalassinoides cf. suevicus, that opportunity of examining a number of cores, to Art Donovan (Exxon, Houston), George Pemberton (Edmonton), Wang Guanzhong followed, forms an irregular unlined maze giving irregular U (Jiaozuo Mining College, China) and M. Martin (Manchester) for and W outlines in section (Figs 1F and 8A). Openings to the field discussion, to S. Tracy for logs of the Upnor Section, to P. Perry erosion surface are about 10 mm diameter, though the burrows and V. Harvey (University of Reading) for reference to unpublished are generally 20mm diameter with depth about 100mm. In theses and, particularly, to R. Bromley (Copenhagen) and A. Curran plan the system is irregular with common short, blind burrows. (Smith College, Mass. U.S.A.) for their constructive criticism of the No wall scratch marks have been observed. T. cf. suevicus text. We thank Arnold and Sons Ltd of Leighton Buzzard for granting suggests the communal burrow system of Alpheus heterochaeles access to their pits in the Woburn Sands; to Berkshire County Council from Georgia estuaries (Howard & Frey 1985, fig. 9B; Bromley for access to Chavey Down Landfill site, Bracknell; J. Harrison & Frey 1974, fig. 8). (Ibstock Warner Ltd) for access to Knowl Hill Quarry; C. J. Patman Colonization by the post-omission suite followed aggra- for access to the Lower Upnor site, and to C. J. Puffett (Multi-Agg Ltd) dation by white and unstained fine-grained sand, infilling and Oxfordshire County Council for access to Shellingford and irregularities in the omission surface (Fig. 8A) with cross- Stanford quarries. University of Reading PRIS Contribution No. 236. lamination passing up into parallel-laminated sand with oc- casional Conichnus and, locally, Skolithos (2mm diameter) Appendix: systematic ichnology and rare Palaeophycus and Rosselia?. This is followed by a Macaronichnus-Ophiomorpha, + P. montanus Ichnofabric 2a Conichnus conicus Mayannil, 1966 (Fig. 5A) (Fig. 8B). The Ophiomorpha shafts are generally normal to the Description. Conical or bluntly pointed subcylindrical structures ori- cobble bed and may extend downwards through the omission ented perpendicular to lamination. Indistinctly or thinly lined mar- surface to terminate unlined, in the humic-rich sand (Fig. 1F). gins which cross-cut bedding, and with a base which tapers to a smooth Macaronichnus suggests a shoreface depositional environment, rounded but distinct apex. Width 30-50 mm, vertical extent of retrusive but following so closely above an erosional surface the water section up to 200 mm which may lead to a slightly conical cylinder up depth could have been shallower. This inference is supported to 100 mm length, representing the burrow cast. Infillings may be con- by the intermediate presence of Conichnus and Skolithos in an cave-up retrusive laminae, chevron-shaped laminae or disturbed zone overall deepening sequence. with a crudely concentric or cone-in-cone fabric. Preseved as endichnia Between the Macaronichnus horizon and the upper cobble in laminated or cross-laminated sand so features of basal surface not bed (base of Barton Clay, Bristow et al. 1991; Fig. 8, horizon 1) observed. cross-lamination is prominent associated with local pebble layers, scattered Ophiomorpha and P. montanus. A multi-stage Occurrence. Ichnofabric 3a, associated with parallel-laminated, or transgression is indicated. weak small-scale cross-lamination, occasionally in Ichnofabric 3c.

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A Typical core offshore estuary estuary shoreface sand wave point bar channel fill 1 2 3a 3c

2m 2m ,oim lOcm j,~>o \ olp~ I I

".'"" __ . 7"liil.

B Ichnofabric/facies boundary upper erosional ? pa leoso Is/---truncation lower gradat. ? ? erosional C Ichnofabric / i

:~C x ~ "a ,'" ------~ _.

D lchnofabric constituents 1 10 100% I i I ,, I J J i I ,| I i I

S , ,,

....

" , c>\\ "c I! ::lo cm

E Colonization window months hours irregular rarely closed years days

Fig. 9. Summary diagram to illustrate relationships between typical core expression, type of boundary between ichnofabrics/facies, typical ichnofabrics and their constituents and nature of the colonization window.

Discussion. These discrete bluntly-rounded concial structures with as the resting, dwelling and adjustment structures of anemone-like ani- concave upwards meniscus infill compare closely with C. conicus as mals and, as such, indicate a marine depositional environment. defined by Pemberton et al. (1988) and described by several workers Conichnus seems to be associated with parallel-lamination and only from similar laminated sand facies of various ages in North America rarely occurs with small-scale cross-lamination. Only small differences (e.g. Cretaceous, Utah, Frey & Howard 1981, fig 2E; Miocene, Den- in grain size and hydraulic regime would seem to negate its occurrence. mark, Radwanski et al. 1975) Pleistocene, N. Carolina, 'Actinarian burrows' Curran & Frey 1977; Pleistocene, Miami and Bahamas Shinn Macaroniehnus segregatis Clifton & Thompson, 1978 1968, pl. 112; Grand Cayman, Conichnus conicus Jones & Pemberton (Fig. 4A-C) 1989, Fig. 10A; D'Alessandro & Bromley 1986, Fig. 14). These biogenic structures can easily be confused with inor- Description. Intrastratal, irregularly meandering burrow structures, ganic V-shaped or chevron-shaped compaction structures in horizontal or gently inclined to lamination and circular in cross- laminated sand formed by collapse of Ophiomorpha shafts or section. Burrow structure diameter 8-10mm, maximum observed galleries, or water escape. The major distinction is the U- burrow length 30-50mm but limited by vertical perspective. Trace shaped rather than V-shaped internal laminae, rounded base, fossil outlined by thin concentrations of dark material (c. 0.5mm limited vertical extent and lateral frequency at horizons thick) in pale coloured sand matrix but mantle is less obvious in the clearly dissociated from Ophiomorpha. dark muddy sand matrix of heterolithic facies. Burrows filled with By analogy with the structures formed by modern sand- light coloured sand, texturally similar but cleaner than host dwelling actinarians (Shinn 1968) these traces are interpreted sediment. Burrow structures frequently densely packed when

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crossovers and interpenetrations occur but true branching has not been Shaft restriction (Fig. 1A, B, E). A short restriction to the burrow observed. opening of Callianassa major is a well-known feature on sandy beaches The trace fossil occurs in two contrasting fabric types (Ichnofabrics (Frey et al. 1978). In Ichnofabric 3a and occasionally in Ichnofabric 1 the 2a and 2b, above) which reflect differences of host sediment, degree of whole length of the pellet-lined shaft (up to 0.7 m) is restricted by the bioturbation and associated trace fossils rather than burrow structure addition of arched laminae 2-3mm thick of muddy coarse silt morphology. In 20-30 cm thick units of slightly muddy or glauconitic (Fig. 1B,E) (distinct from the pellets of the wall lining) to leave a siphun- cross-stratified quartz sand the trace fossils form 15-30% of the fabric cular-like tube (3-5 mm diameter) that expands rapidly as the base of (Bioturbation Index (BI) 2; Taylor & Goldring 1993) associated with the shaft curves into a gallery or drops further (Fig. 1E). Restriction is Ophiomorpha and Skolithos with or without Planolites (Fig. 4C). The present when shaft and gallery are of contrasting diameters and when a large-scale cross-stratified sand wave facies of the Woburn Sands is gallery (inside diameter) may be half again the diameter of the associ- densely bioturbated (up to 60%, BI 3) with Macaronichnus which is ated shaft (inside wall diameter). Although it has not been possible to frequently horizontal, and markedly discordant to the dip of the trace out a complete burrow system or even a gallery into a second shaft, bedding (Fig. 4A, B). Ophiomorpha are rare in this fabric but groups of when traced along a single sand interval, about 50% of shafts are restric- small sand-filled P. montanus and large mud-filled P. beverleyensis ted. What appears to be the same restriction was described by Frey et al. occur locally. (1987, figs 29, 30) from Korean tidal flats and interpreted as a passive filling through which a new aperture had been punched. But in our Discussion. These burrow structures are similar to specimens of M. material the lining is laminated and composed of muddy silt, quite segregatis described in detail by Clifton & Thompson (1978) and Cur- distinct from the sand normally plugging the burrow or the central tube. rran (1985), although the ichnotaxonomic assignment of Curran's An alternative explanation (which may also apply to the shafts descri- specimen to Macaronichnus is questioned (see Bromley 1990, p. 179). bed by Frey et al. 1987) is that restriction along the whole length of the The wall is less prominent, possibly slightly thinner, although the shaft would facilitate the occupier in creating an upward draft, since flow burrows described here have only been studied in vertical section. The velocity in pipes is proportional to cross-sectional area. Shinn (in Frey et burrow diameter is similar to Curran's Cretaceous specimens but ai. 1978, fig. 8B) interpreted the local short restriction along the larger than those described by Clifton & Thompson (3-5 mm) from a exhalent shaft as helping prevent entry of extraneous material and pos- variety of localities and horizons. The observed length of 30-50 mm is sibly predators into the burrow. No such restriction has been observed in much shorter than the 200 mm of Curran's specimens but again is a the present material. Frey et al. ( 1978, fig. 4d) figured detached burrow function of the limited vertical perspective. fragments with a concentric wall lining from 18 m water depth which The ichnotaxonomic problem in the distinction of these burrow show resemblance to the present material. structures from the closely similar ichnotaxa Planolites and Palaeo- The restriction is reminiscent of restriction in the burrows of phycus has been discussed by several authors (Clifton & Thompson Rhizocorallium or of cephalopod cameral fillings, both described by 1978; Curran 1985; Fillion 1989). Considering the criteria formulated Seilacher (1968) and attributed to passive flow and deposition of mud by Pemberton & Frey (1982) of wall lining, active or passive filling through the burrow, or through a broken cephalopod. Absence of the (similar or different to the host sediment), we conclude that these struc- restriction in galleries and the septum-like form of the restriction tures should be assigned to Macaronichnus rather than Planolites. It is shows that the restriction here is of different origin. But it may be the discrete wall that separates Macaronichnus from Planolites (which speculated whether restriction was caused by the animal that created is unlined), and the infiil cleaner than the matrix, the result of the the burrow or by a subsequent occupier; possibly of a different taxon. feeding activity of the producer, thus contrasting with the passive infill We tentatively suggest that this type of Ophiomorpha was formed by of Palaeophycus. The association with contrasting, usually smaller, relocated animals. unlined sand or mud-filled Planolites further reinforces this distinc- tion. However, as pointed out by Fillion (1989) diagenetic effects can Shaft opening andcohmization. Truncation has in most cases removed the artificially enhance a wall which is difficult to observe in the dark original opening to the burrow except in Ichnofabric 3b, but we envisage heterolithic muddy sand matrix. that in each ichnofabric it originally opened above a muddy lamina or We agree with previous authors' interpretation of Macaronichnus as laminae where it was restricted. In Ichnofabric 3b colonization appar- the burrow structure of a deposit feeding organism, probably a marine ently took place on a muddy surface during an interval of relatively low polychaete (Clifton & Thompson 1978; Curran 1985; Bromley 1990; hydraulic intensity. In modern carbonate environments many of the Saunders & Pemberton 1990). The occurrence in cross-stratified sands responsible for Ophiomorpha and Thalassinoides open to which are bioturbated virtually penecontemporaneously with deposi- wide funnels, often associated with sediment mounds. But in these tion, also confirms the high energy environment for these burrows tropical situations the hydraulics are quite distinct. suggested by previous workers. However, the sedimentology of the host sediment suggests formation in subtidal bars or sand waves (Curran Pellet (wall) lining. Ophiomorpha shows much variation in the nature of 1985) rather than lowermost intertidal or beach environments sug- the pelletal wall lining from unlined, roof lined to fully lined. The gested for modern analogous organisms (Clifton & Thompson 1978; lining may be very thick and can also vary along a burrow (Frey et al. Saunders & Pemberton 1990). 1978). We have also observed much variation in the nature of the Ophiomorpha nodosa Lundgren, 1891 (Figs 1, 2, 4, 5, 7 & 8) pellets from globular to muddy and/or rich in plant debris, when the pellet has generally been deformed (see Frey & Howard 1975, p. 289 Description. The material agrees with the diagnosis given by Frey et al. referring to Farrow 1971). Deformed pellets may resemble O. irregu- (1978). Although the specimens exhibit a broad range of diameter and laire (Frey et al. 1978) but examples we have observed that might be other aspects, discussed below, we do not consider that further attributed to this taxon can be shown to be compressed muddy or plant taxonomic division is justified. rich pellets. The loosely winding burrow system of '0. irregulaire' has been observed in the Corallian, U. Jurassic (above the Bencliff Grit, Preservational variation. Variation in the preservation of Ophiomorpha Upper Jurassic, Osmington Mills, Dorset, shore section SY 740816; has been frequently noted in the literature (Frey et al. 1978), often Ffirsich 1974) where the Ophiomorpha-burrower was apparently re- followed by discussion as to the taxonomic significance. But it is now stricted in downward penetration by a mud layer. generally agreed that where the burrow is predominantly pellet-lined then Ophiomorpha should be applied to the whole trace. We have Blind ended burrows. Several Ophiomorpha in the section at Hen- followed this convention but difficulties arise when, for example, a gistbury Head (above) extend over 0.7 m down through post-omission discrete unlined shaft is present. sediment and then penetrate the humic rich sand (Figs. 1F and 8). A

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lined shaft continues 0.15 m into the humic sand and then becomes ALPERT, S. P. 1974. Systematic review of the genus Skolithos. Journal of Paleon- unlined and somewhat twisted before terminating abruptly. Such tology, 48, 661-669. ARMSTRDNO, L. A., TEN HAVE, A. & JOHNSON, H. D. 1987. The geology of the burrows probably represent failed colonization. In Ichnofabric 3a and, Gannet Fields, Central North Sea, U. K. Sector. In: BRooKs, J. & GLENNIE, especially 3b the majority of burrows terminate blindly suggesting K. W. (eds) Petroleum Geology of North West Europe. Graham & Trottman, incomplete formation of a burrow system due to constraint in the time London, 533-548. available for burrow construction, the short colonization window. ARKELL, W. J. 1947. The Geology of Oxford. Oxford University Press, Oxford. ASGAARD, U. • BROMLEY, R. G. 1974. Sporfossiler fra den Mellemmiocaene transgression i S~by-Fasterholt-Omradet. Dansk geologisk Forening, Planolites montanus Richter, 1937 (Figs 2, 3, 4, 7 & 8) ,4rsskrift for 1973, 11-19. BOCKEL1E, J. F. 1991. Ichnofabric mapping and interpretation of Jurassic reser- Remarks. P. montanus occurs as fine (1-2 mm diam.) winding, dark and voir rocks of the Norwegian North Sea. Palaios, 6, 206-215. somewhat discontinuous fecal strings of sandy mud either, more or less BOSENCE, D. W. J. 1973. Facies relationships in a tidally influenced environment. evenly distributed, or concentrated within, around and through O. Geologie en Mijnbouw, 52, 63--67. nodosa. There is rather greater prominence of vertical elements than is BowN, T. M. 1982. Ichnofossils and rhizoliths of the nearshore fluvial Jabel Quetrani Formation (Oligocene), Fayum Province, Egypt. Palaeogeogra- usually described in the literature. Kennedy & Sellwood (1970, p. 108) phy, Palaeoclimatology, Palaeoecology, 40, 255--309. and Bosence (1973) noted the local concentrations associated with BmlX;ES, P. D. 1982. Ancient offshore tidal deposits. In'. STmOE, A. H. (ed.) Ophiomorpha. Frey & Howard (1975) described burrows from resin Offshore tidal sands. Chapman & Hall, 172-192. castings associated with enlarged (brood) chambers of Upogebia affinis B~STOW, C. R., FI~SHNEY, E. C. & PENN, I. E. 1991. Geology of the country which resemble the present burrows but the similarity is superficial around . Memoir of the British Geological Survey (England since the burrows, attributed to the activity of U. affinis larvae were and Wales), Sheet 329. BROMLEY, R. G. 1990. Trace fossils: biology and taphonomy. Unwin Hyman, open, whereas here Planolites was a fecal casting and could not have London. been cast in resin. -- & EKDALE, A. A. 1986. Composite ichnofabrics and tiering of burrows. In Ichnofabric 1 Planolites montanus is generally evenly distributed Geological Magazine, 123, 59~5. and only occasionally clustered within and around Ophiomorpha. It is -- & FREY, R. W. 1974. Redescription of the trace fossil Gyrolithes and also occasionally present in Ichnofabric 3 as described by Kennedy & taxonomic evaluation of Thalassinoides, Ophiomorpha and Spongeliomor- Sellwood (1970, p. 108) and in Ichnofabric 2 (Curran 1985; Bosence pha. Bulletin of the Geological Society of Denmark, 23, 311-335. BROWN, S. 1986. Jurassic. In: GLENNIE, K. W. (ed.) Introduction to the Petroleum 1973). Geology of the North Sea. (2nd edn). Blackwell, London, 131-159. BUCK, S. G. 1987. Facies and sedimentary structures of the Folkestone Beds (Lower Skolithos isp. Haldeman, 1840 Greensand, early Cretaceous) and equivalent strata in southern England. PhD thesis, University of Reading. Description. Vertical to steeply inclined, generally unbranched and CHAMBERLAIN, C. K. 1975. Recent Lebensspuren in nonmarine aquatic environ- generally cylindrical burrows traceable for up to 30 cm. Burrow wall ments. In" FREV, R. W. (ed.) The study of Trace Fossils. Springer, New York, 431-458. generally indistinct, occasionally thinly lined. Two size associations CUFTON, H. E. & THOMPSON, J. K. 1978. Macaronichnus segregatis: a feeding are typical: (a) a slender straight to slightly sinuous burrow of only a structure of shallow marine polychaetes. Journal of Sedimentary Petrology, few agglutinated grains diameter, generally in dense aggregations, (b) 48, 1293-1302. slender burrows with a smooth wall 2-Smm diameter occurring singly C~MES, T. P. 1977. Trace fossils of an Eocene deep-sea fan, northern Spain. In: or aggregated. CreMES, T, P. & HARPER, J. C. (eds) Trace fossils 2. Geological Journal, Special Issue, 9, 71-90. -- , GOLDRING, R., HOMEWOOD, P., VAN STUIJVENBERG, J. ~£ WINKLER, W. Discussion. The shafts fall into the fairly wide range of this ichnotaxon 1981. Trace fossil assemblages of deep-sea deposits, Gurnigel and Schlieren (Alpert 1974). But the trace is mostly only observed under particular flysch (Cretaceous-Eocene), Switzerland. Eclogae geologica Helvetica, 74, weathering conditions where the sand is dry and has been lightly sand- 953-995. blasted by wind. Under such situations dense aggregations are seen to CURRAN, H. A. 1985. The trace fossil assemblage of a Cretaceous nearshore be associated with event-bed colonization. Curran (1985) described environment: Englishtown Formation of Delaware, U.S.A. In : CURRAN, H. A. (ed.) Biogenic structures: their use in interpreting depositional environments. somewhat more sinuous and occasionally branched burrows in similar Society of Economic Paleontologists and Mineralogists, Special Publica- ichnofabric which he compared with S. pusillus (Frey & Howard 1982). tion, 35, 261-276. Clusters of similar delicate branching burrows were recorded by Mar- -- & FREY, R. W. 1977. Pleistocene trace fossils from North Carolina (U.S.A.), tino & Curran (1990). The type a burrows suggest formation by slender and their Holocene analogues. In: CRIMES, T. P. & HARPER, J. C. 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Palaios, 6, 232-249. -- ,-- & PEMBERTON, S. G. 1984. Ichnology." trace fossils in sedimentology and ?S.pusillus occur at several levels often densely aggregated and associ- stratigraphy. Society of Economic Paleontologists and Mineralogists, Short ated with P. montanus. In the estuarine point bar facies cm-dm units, Course Notes 15. interpreted as units of rapid deposition, display thread-like burrows a FARROW, G. E. 1971. Back-reef and lagoonal environments of Aldabra Atoll few grains thick which may weather out, but S. linearis is more typical. distinguished by their burrows. Symposium of the Zoological There is a strong possibility of confusion with unlined shafts of Society of London, 28, 455-500. Ophiomorpha. EILLION, D. 1989. Les critbres discriminants a l'int6rieur du trityque Palaeo- phycus-Planolites-Macaronichnus. Essai de synth~se d'un usage critique. Comptes rendus des Seances de l'Academie des Sciences, 309, 169--172. References Flay, R. W. 1990. 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Received 21 April 1992; revised typescript accepted 17 July 1992.

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