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Trace-Fossils and the Sedimentary Surface in Shallow-Water Marine Sediments
TRACE-FOSSILS AND THE SEDIMENTARY SURFACE IN SHALLOW-WATER MARINE SEDIMENTS R. GOLDRING
Department of Geology, University of Reading, Reading (Great Britain)
INTRODUCTION
In three papers, SEILACHER (1954, 1955, 1958) has distinguished two broad assemblages of trace-fossils (ichnofossils) in marine deposits. One may be associated with what are considered to be shallow-water sediments, and the other with what are considered to be relatively deeper-water sediments, in particular turbidites. In the broadest synonymy, one is associated with a flysch facies; the other with shallow marine facies. Few ichnogenera are common to both facies. The turbidite facies is characterised by a preponderance of grazing traces (Pascichnia), whilst resting traces (Cubichnia) and dwelling burrows (Domichnia) are rare or absent. The two latter are well represented in epicontinental sediments. Besides these two distinct trace-fossil assemblages, it is considered that the trace assemblages associated with shallow-water and shelf sediments may be usefully divided into two groups. Although the sedimentological characters of each are substantially distinct, a consideration of the trace assemblages may add useful information towards an appreciation of the sedimentary environment. In analysing epicontinental assemblages, Seilacher has drawn on such examples as the Mesozoic sandstones of Wiirttemberg and the Cambrian of the Salt Range, which include resting traces, often aligned, and trails made on, from, or very close to the sea floor. Migrating burrows constructed below the surface, such as Teichichnus, Ptiycodcs and Rhizocorallium, together with other feeding burrows, are also present. The sea floor must have been quite rapidly covered with accumulating sediment, for those traces, made close to it, to have been preserved. Seldom, if at all, was it subjected to any considerable penecontemporaneous erosion. Vertical repetitions of Asteriacites lumbricalis (SEILACHER 1953, p.100) in sandstone showing primary current lineation, indicate a fairly continuous accumulation of about 2 cm, and associated upward migrating (retrusive) Diplocraterion sp. of at least 15 cm. I n contrast, there are other sediments which, from their sedimentological characteristics and associated body-fossils, must also be considered as epicontinental and neritic. These rarely, if ever, yield any trace-fossil, in position of formation, that was made on or immediately below the sedimentary surface. This does not necessarily mean that such markings were not made in these sediments, but that if they were, then they were never fully preserved.
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CH 0NDR ITES
The trace-fossil Chondrites has been redescribed by SIMPSON(1957). It is known from the Ordovician to the Tertiary, and occurs in several types of preservation (Fig.1). With bed-junction preservation, a change in the nature of the sediment accumulating takes place, and the new sediment fills in the tunnel system excavated by the organism. The lithological change may be gradual, and without penecontemporaneous erosion. If there is no sedimentary change, only subsequent diagenetic differentiation between the sediment filling the tunnels and the matrix, will render them observable. Sharp changes in lithology, as, for instance, between a sandstone and a shale, may be associated with slight penecontemporaneous erosion removing portions of the proximal shafts, and give rise to concealed bed-junction preservation. Burial preservation is where the sediment surrounding the mucus-lined tubes has been winnowed away leaving a pile of Fucus-like strands, indicating definite penecontemporaneous erosion, and the removal of the sedimentary surface from which the tunnel system was initiated. In the latter two types, it is not however possible to measure the amount of erosion that has taken place. Bed-junction preservation and diagenetic preservation were noted by Simpson to occur abundantly i n the Lower Lias of the south coast of England, but burial preservation is absent. Burrowing pelecypods and Pinna in their attitude of growth, and Lima also in the growth position, with encrusting oysters on the posterior part of the shell (HALLAM,I960), also show the infrequency of penecontemporaneous erosion. In the Carboniferous limestone of the Bristol district, only a few examples of burial preservation have been found. Bed-junction preservation greatly predominates, such as cylinders of limestone piping downwards from a limestone into an argillaceous band, and vice versa, but diagenetic preservation is also present in the calcite-mudstone bands. In contrast, in the Upper Emsian Lynton Beds and the Givetian Ilfracombe Beds of southwest England, concealed bed-junction preservation predominates in the slate bands, and burial preservation is not infrequent. Penecontemporaneous erosion was of frequent occurrence. The Lynton Beds and the Ilfracombe Beds are neritic sediments, and both are conformably underlain by continental deposits. Other trace-fossils have not yet been described from the beds but, so far as the writer is aware, no exogenic forms occur. The type of preservation of Chondrites in these sediments contrasts sharply with that in the Dinantian, Lias, and elsewhere in the Mesozoic (SIMPSON, 1957, p.490).
THE UPPER DEVONIAN BAGGY BEDS
The Baggy Beds of southwest England stratigraphically lie in a conformable sequence between continental and neritic sediments. They almost completely lack surface-made traces and shallow burrowings are only locally abundant. DiplocrationJoyo,
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a migratory U-burrow, vertical to the bedding, is the most common trace-fossil. It occurs in a banded lithology of fine-grained sandstone and mudstone; the laminae are from 0.1-1.0 cm thick. This lithology accounts for approximately 15-20% of the Baggy Beds (400 m). GOLDRING (1962) has interpreted the behaviour pattern of this species as a response to repeated phases of sedimentation and erosion. Both retrusive and protrusive forms are present. Only a few examples of what are considered to be the actual aperture to the burrow are known, and in the majority of specimens, which are retrusive forms, only a remnant of the original structure is preserved beneath a sandstone capped erosion surface. In spite of the overall accumulation of sediment, there must have been repeated phases of degradation of the sedimentary surface. The overall amount of erosion is not easy to determine, but was generally in the order of 5-10 cm for most specimens, whilst individual phases were probably less than 1 .O cm. I n nearly every case where the species occurs, erosion of sediment can be deduced, with at least twice the amount of sediment preserved having been originally deposited. Only organisms which were able to penetrate to a depth of 5-10 cni or more, were likely to leave any trace of their existeiice in the fossil record. The open U-tube Arenicolites curvatus is present in more massive sandstone. The tubes must have been frequently subjected to erosion, as fragments commonly occur in interbedded congloFig.1. The amount of sedimentation or erosion as indicated by the adjustment to depth and mode of preservation of various species. Heights of solid arrows show amount of sedimentation or erosion. ( I ) Movement pattern of burrowing pelecypod with single siphonal opening e.g., Mya (after REINECK. 1958). With stationary sedimentary surface (a) growing organism burrows deeper. With rapid sedimentation (b) organism migrates towards surface, leaving infilled burrow of same breadth as shell. With degradation of surface (c) organism migrates downwards, leaving burrow of same breadth as shell. (2) Movement pattein of polycheate worm, e.g., Nereis (after REINECK, 1958). Older colonised surface ( a ) is rapidly covered by sediment ( b and c) and during deposition of sediment, paths of escape are directly upwards. New colonisation surface (c) with irregular burrows. Burrows in ( a ) and ( c ) generally mucus lined; in (b) unlined. Similar pattern observed by author in sands of Bagshot Beds (Eocene) at Bramshill (SU 756610). (3) Movement pattern of organism dwelling in single tube, e.g.. Cerianthus (after SCHAFER, 1956). With sedimentation, the coral moves upwards leaving an infilled burrow. A similar pattern might be expected in traces attributed to Skolithos and Monocraterion. ( 4 ) Movement of Asteriocites lunibricolis (after SEILACHER, 1953): (a) with sedimentation the ophiuroid migrates upwards in stages (i-iii), and ( b ) combined plan of all impressions. ( 5 ) Preservation pattern of Chondrites (after SIMPSON, 1957). The tunnel system (a) is infilled (b) following a change in the type of sediment being deposited (bed-junction preservation). Slight degradation of the surface (c) removes the proximal shafts before further sediment, of a different type, accumulates (concealed bed-junction preservation). Renewed degradation of the surface winnows away the sediment, leaving the mucus-lined infilled tunnels as burial preservation (d). (6) Preservation pattern of Arenicolites curvatus (after GOLDRING, 1962). The sedimentary surface with the open U-tubes (a) has been degraded (b), the mucuscemented tube fragments accumulating in an intraformational conglomerate; sediment has filled the tubes in (b). (7) Movement pattern of Diplocration yoyo (after GOLDRING, 1962). In the Upper Devonian Baggy Beds of north Devon, this trace occurs in the various types shown in (F),where all have been truncated to a common erosion surface. It is considered that repeated phases of erosion and sedimentation led to the development of the various types. Stage (A), development of burrow ( I ) . With degradation of the surface, this tube migrates downwards, and at intervals, new tubes (2 and 3) are constructed ( E and C ) . Sedimentation follows (Dand E) but some of the tubes are abandoned. Stage ( F ) , all tubes are abandoned and erosion reduces them to a common base.
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merates. It is not possible to determine how much of the tubes were eroded, though there are fragments up to 5 cm in length. None of the body fossils (thick-shelled pelecypods and gastropods) occur in their growth position. It is interesting to attempt a determination of what the sedimentary surface must have often been like in Baggy times. From a freak preservation at one locality, it can be deduced that the sedimentary surface locally carried a prolific number of DFlocraterion. Judging from the large number of specimens of Lingula, frequently wellpreserved, lying on bedding surfaces elsewhere in the Baggy Beds, other sedimentary surfaces must often have been extensively burrowed by this brachiopod, though not to the depth to which Diplocraterion penetrated. There were probably also banks of the thick-shelled pelecypod Dolabra (Arcacea), none of which have been preserved, the specimens occurring only in cross-bedded sandstone units. If local areas were at times, quite highly populated with surface-dwelling and burrowing organisms, this would indicate (REINECK, 1958), so far as these areas are concerned, periods of only slight sedimentation and erosion. Such areas if they existed at the present day would likely be quite readily observable (e.g., tidal flats) in contrast with areas, such as channels where rapid changes may occur. Scarcely any surface that could represent such an area has been preserved.
AN INTERMEDIATE ASSEMBLAGE
The Baggy Beds and the basin facies of the Blue Lias, or the Lower Jurassic of Wiirttemberg, may be considered as the two extremes for the preservation of sedimentary surfaces. What might be considered as intermediate members, are sections of the Carboniferous of northern England and southern Scotland. There, although there is a good deal of evidence of penecontemporaneous erosion, nevertheless sedimentary surfaces with fossils in their growth positions, and surface-made traces, are not infrequently preserved. The upper part of the Carboniferous limestone and the succeeding Yoredales (Visean to Namurian), together with their Scottish equivalents, comprise repetitive successions of limestone, marine and non-marine shale, sandstone and occasional coals. In palaeoecological studies on the marine shales, CRAIG (1952, 1 9 5 4 , 1956) and FERGUSON (1962) have shown that, occasionally, surface-living organisms such as Productus concinnus and pectiniform pelecypods, and more frequently burrowers such as Lingula and Edrnondia, occur in their position of growth. However, sedimentary scour structures and the occurrence of concentrations of disarticulated and fragmentary shells, show that the sedimentary surface was always likely to be swept by currents. The trace-fossils associated with the series are not well documented, and have often been described merely as “fucoids”. DONALDSON and SIMPSON (1962) have described an association from the Visean (D) limestones of north Lancashire. Besides numerous surface-made, exogenic trails, one bed is covered by conical mounds of castings, Cliornatichnus, above vertical burrows, pushed to the surface by an unknown organism. Judging by the density of the castings (about
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30/m2), they must have been formed on a sedimentary surface of some duration. They were then rapidly covered, and no reworking took place. Spirophyton, Cock’s tail, markings may be connected with the castings, and there are also endogenic burrows, some attributable to Teichichnus and Chonrirites. Cauda-galli markings are common elsewhere in the series, and probably represent the central laminated trace of a sediment-eating organism, migrating through the sediment close to the sea-floor. A Corophium-like burrow with thick walls, near to Tisoa siphonalis MARCEL DE SERRES (BATHER1924), which extends about 5 cm into shale, is common at Earlsferry, Fife, in the Carboniferous Limestone Series. Footprints have been recorded from the Calciferous sandstone (SMITH,1891). In general, the faunal, algal and lithological association indicate a fairly shallow-water sedimentary environment for those parts of the cyclothems considered as marine.
CONCLUSIONS
These analyses, albeit brief, of trace-fossil assemblages formed in no great depth of water, lead to three main conclusions. Firstly, two distinct types of assemblages are represented, possibly the end members of a continuous series, (1) with surface or near-surface made traces, as well as traces made at greater depths, and (2) with only traces made some depth below the sedimen tary surface. Secondly, on the one hand, the frequent occurrence of abundant surface-made traces, or those made very close to the sedimentary surface, indicate fairly continuous sedimentation, and the preservation of nearly every sedimentary surface. On the other hand, the complete absence of these trace types, associated with frequent evidence of erosion, is testimony of discontinuous sedimentation. and the eventual preservation of only a fraction of the actual sediment deposited. REINECK (1960) has shown that only 1/lO,OOO-I/lOO,OOO part of the sediment deposited i n recent shallow water sediments in tidal seas, is actually fossilised. Similar conditions must have existed during the deposition of much of the Baggy Beds. Thirdly, the adjustments of certain organisms attempting to maintain a constant depth below the sedimentary surface, can be used to estimate the amount of sedimentation or erosion that has taken place. Also, the mode of preservation of others is an indication of this. REINECK (1 958) has shown the effect of slow and fast sedimentation and erosion on Mya, Nereis and other organisms in recent sediments. Vertical repetition of Asteriacites and retrusive Diylocraterion give an indicatioii of the amount of sedimentation. These and other examples are shown diagrammatically in the accompanying text-figure. The sedimentological causes for the division are probably complex. To permit continuous sedimentation with only limited penecontemporaneous erosion, the environment must have been under conditions leading t o continuous aggradation. If sediment accumulated beyond a certain rate, colonisation by burrowing organisms would be
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impossible, and most existing organisms would attempt t o migrate elsewhere (REINECK, 1958; MIDDLEMISS, 1962). Discontinuous sedimentation at the present day, is particularly well developed in the intertidal zone of continental shelves, and is a typical feature of prodelta sediments.
SUMMARY
Two distinct trace-fossil assemblages from shallow-water marine sediments are recognised: (a) with traces made on, or close to the sedimentary surface as well as those penetrating deeper, and (h) with only traces made well below the sedimentary surface. Whereas there is evidence for only limited penecontemporaneous erosion in sediments associated with the former type, there is evidence of considerable and frequent penecontemporaneous erosion associated with the latter. The amount of sedimentation and erosion can be estimated from the behaviour pattern and mode of preservation of certain trace-fossils.
REFERENCES
BATHER, F. A,, 1924. Tisoa siphonalis MARCEL DE SERRES, a supposed Liassic annelid. Naturalist, 1925 : 7-10. CRAIG,G. Y., 1952. A comparative study of the ecology and palaeoecology of Lingula. Trans. Edinbrrrgh Geol. SOC.,15 : 110-120. CRAIG,G . Y , 1954. The palaeoecology of the Top Hosie Shale (Lower Carboniferous) at a locality near Kilsyth Quart. J . Geol. SOC.London, 110 : 103-1 19. CRAIG,G. Y.,1956. The mode of life of certain Carboniferous animals from the west Kirkton Quarry, near Bathgate. Trans. Eilinbirrgh Gcol. Soc., 16 : 272-219. DONALDSON, D. and SIMPSON, S., 1962. Choniatichtius. a new ichnogenus, and other trace-fossils of Wegber Quariy. Liverpool Manrhester Geol. J., 3 : 73-81. FEROUSON, L., 1962. The palaeoecology of a Lower Carboniferous marine transgression. J. Paleontol., 36 : 1090-1101. GOLDRING, R., 1962. The trace fossils of the Baggy Beds (Upper Devonian) of North Devon, England. Paliiontol. Z., 36 : 232-257. HALLAM, A., 1960. A sedimentary and faunal study of the Blue Lias of Dorset and Glamorgan. Phil. Trans. Roy. SOC.London, Ser. B, 243 : 1 4 . MIDDLEMISS, F. A,, 1962. Vermiform burrows and rate of sedimentation in the Lower Greensand. Geol. Mqy.. 99 : 3 3 4 0 . REINECK,H. E., 1958. Wuhlbau-Gefiige in Abhangigkeit von Sediment-Umlagerungen. Senckcviber,niana Lethaca, 39 : 1-23, 54-56. REINECK, H. E.. 1960 Uber Zeitliicken in rezenten Flachsee-Sedimenten. Geol. Rundschair,49 : 149-161. SCHAFER, W., 1956. Wirkungen der Benthos-Organismen auf den jungen Schichtverband. Srtickenbet-yiatia Lethaea, 37 : 183-263. SEILACHER, A., 1953. Studien zur Palichnologie. 11. Die fossilen Ruhespuren (Cubichnia). Neites Jahrb. Geol. Palaontol. Abhandl., 98 : 87-124. SEILACHER, A., 1954. Die geologische Bedeutung fossiler Lebensspuren. Z. Derrt. Grol. Ges., 105 : 2 14-227. SEILACHER, A,, 1955. Spuren und Fazies irn Unterkambrium. In: 0. SCHINDEWOLF und A. SEILACHER, BeitrLige zur Kenntnis des Kambriirrns in der Salt Range (Pakistan) - A k a d Wiss. Lit. Mainz Abhandl. Math. Nut. KI., 1955 : 313-399.
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SEILACHER, A., 1958. Zur okologischen Charakteristik von Flysch und Molasse. Eclogae Ceol. Helv., 51 : 1062-1078. SIMPSON, S., 1957. On the trace-fossil Clionrlrites. Quart. J . Geol. SOC.London, 112 : 475-500. SMITH,J., 1891. Note on the occurrence of footprints in the calciferous sandstone between West Kilbride and Fairlie. Trans. Geol. SOC.GlaTyow, 9 : 201-203.
Department of Geology, University of Reading, Reading (Great Britain)
INTRODUCTION
In three papers, SEILACHER (1954, 1955, 1958) has distinguished two broad assemblages of trace-fossils (ichnofossils) in marine deposits. One may be associated with what are considered to be shallow-water sediments, and the other with what are considered to be relatively deeper-water sediments, in particular turbidites. In the broadest synonymy, one is associated with a flysch facies; the other with shallow marine facies. Few ichnogenera are common to both facies. The turbidite facies is characterised by a preponderance of grazing traces (Pascichnia), whilst resting traces (Cubichnia) and dwelling burrows (Domichnia) are rare or absent. The two latter are well represented in epicontinental sediments. Besides these two distinct trace-fossil assemblages, it is considered that the trace assemblages associated with shallow-water and shelf sediments may be usefully divided into two groups. Although the sedimentological characters of each are substantially distinct, a consideration of the trace assemblages may add useful information towards an appreciation of the sedimentary environment. In analysing epicontinental assemblages, Seilacher has drawn on such examples as the Mesozoic sandstones of Wiirttemberg and the Cambrian of the Salt Range, which include resting traces, often aligned, and trails made on, from, or very close to the sea floor. Migrating burrows constructed below the surface, such as Teichichnus, Ptiycodcs and Rhizocorallium, together with other feeding burrows, are also present. The sea floor must have been quite rapidly covered with accumulating sediment, for those traces, made close to it, to have been preserved. Seldom, if at all, was it subjected to any considerable penecontemporaneous erosion. Vertical repetitions of Asteriacites lumbricalis (SEILACHER 1953, p.100) in sandstone showing primary current lineation, indicate a fairly continuous accumulation of about 2 cm, and associated upward migrating (retrusive) Diplocraterion sp. of at least 15 cm. I n contrast, there are other sediments which, from their sedimentological characteristics and associated body-fossils, must also be considered as epicontinental and neritic. These rarely, if ever, yield any trace-fossil, in position of formation, that was made on or immediately below the sedimentary surface. This does not necessarily mean that such markings were not made in these sediments, but that if they were, then they were never fully preserved.
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CH 0NDR ITES
The trace-fossil Chondrites has been redescribed by SIMPSON(1957). It is known from the Ordovician to the Tertiary, and occurs in several types of preservation (Fig.1). With bed-junction preservation, a change in the nature of the sediment accumulating takes place, and the new sediment fills in the tunnel system excavated by the organism. The lithological change may be gradual, and without penecontemporaneous erosion. If there is no sedimentary change, only subsequent diagenetic differentiation between the sediment filling the tunnels and the matrix, will render them observable. Sharp changes in lithology, as, for instance, between a sandstone and a shale, may be associated with slight penecontemporaneous erosion removing portions of the proximal shafts, and give rise to concealed bed-junction preservation. Burial preservation is where the sediment surrounding the mucus-lined tubes has been winnowed away leaving a pile of Fucus-like strands, indicating definite penecontemporaneous erosion, and the removal of the sedimentary surface from which the tunnel system was initiated. In the latter two types, it is not however possible to measure the amount of erosion that has taken place. Bed-junction preservation and diagenetic preservation were noted by Simpson to occur abundantly i n the Lower Lias of the south coast of England, but burial preservation is absent. Burrowing pelecypods and Pinna in their attitude of growth, and Lima also in the growth position, with encrusting oysters on the posterior part of the shell (HALLAM,I960), also show the infrequency of penecontemporaneous erosion. In the Carboniferous limestone of the Bristol district, only a few examples of burial preservation have been found. Bed-junction preservation greatly predominates, such as cylinders of limestone piping downwards from a limestone into an argillaceous band, and vice versa, but diagenetic preservation is also present in the calcite-mudstone bands. In contrast, in the Upper Emsian Lynton Beds and the Givetian Ilfracombe Beds of southwest England, concealed bed-junction preservation predominates in the slate bands, and burial preservation is not infrequent. Penecontemporaneous erosion was of frequent occurrence. The Lynton Beds and the Ilfracombe Beds are neritic sediments, and both are conformably underlain by continental deposits. Other trace-fossils have not yet been described from the beds but, so far as the writer is aware, no exogenic forms occur. The type of preservation of Chondrites in these sediments contrasts sharply with that in the Dinantian, Lias, and elsewhere in the Mesozoic (SIMPSON, 1957, p.490).
THE UPPER DEVONIAN BAGGY BEDS
The Baggy Beds of southwest England stratigraphically lie in a conformable sequence between continental and neritic sediments. They almost completely lack surface-made traces and shallow burrowings are only locally abundant. DiplocrationJoyo,
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a migratory U-burrow, vertical to the bedding, is the most common trace-fossil. It occurs in a banded lithology of fine-grained sandstone and mudstone; the laminae are from 0.1-1.0 cm thick. This lithology accounts for approximately 15-20% of the Baggy Beds (400 m). GOLDRING (1962) has interpreted the behaviour pattern of this species as a response to repeated phases of sedimentation and erosion. Both retrusive and protrusive forms are present. Only a few examples of what are considered to be the actual aperture to the burrow are known, and in the majority of specimens, which are retrusive forms, only a remnant of the original structure is preserved beneath a sandstone capped erosion surface. In spite of the overall accumulation of sediment, there must have been repeated phases of degradation of the sedimentary surface. The overall amount of erosion is not easy to determine, but was generally in the order of 5-10 cm for most specimens, whilst individual phases were probably less than 1 .O cm. I n nearly every case where the species occurs, erosion of sediment can be deduced, with at least twice the amount of sediment preserved having been originally deposited. Only organisms which were able to penetrate to a depth of 5-10 cni or more, were likely to leave any trace of their existeiice in the fossil record. The open U-tube Arenicolites curvatus is present in more massive sandstone. The tubes must have been frequently subjected to erosion, as fragments commonly occur in interbedded congloFig.1. The amount of sedimentation or erosion as indicated by the adjustment to depth and mode of preservation of various species. Heights of solid arrows show amount of sedimentation or erosion. ( I ) Movement pattern of burrowing pelecypod with single siphonal opening e.g., Mya (after REINECK. 1958). With stationary sedimentary surface (a) growing organism burrows deeper. With rapid sedimentation (b) organism migrates towards surface, leaving infilled burrow of same breadth as shell. With degradation of surface (c) organism migrates downwards, leaving burrow of same breadth as shell. (2) Movement pattein of polycheate worm, e.g., Nereis (after REINECK, 1958). Older colonised surface ( a ) is rapidly covered by sediment ( b and c) and during deposition of sediment, paths of escape are directly upwards. New colonisation surface (c) with irregular burrows. Burrows in ( a ) and ( c ) generally mucus lined; in (b) unlined. Similar pattern observed by author in sands of Bagshot Beds (Eocene) at Bramshill (SU 756610). (3) Movement pattern of organism dwelling in single tube, e.g.. Cerianthus (after SCHAFER, 1956). With sedimentation, the coral moves upwards leaving an infilled burrow. A similar pattern might be expected in traces attributed to Skolithos and Monocraterion. ( 4 ) Movement of Asteriocites lunibricolis (after SEILACHER, 1953): (a) with sedimentation the ophiuroid migrates upwards in stages (i-iii), and ( b ) combined plan of all impressions. ( 5 ) Preservation pattern of Chondrites (after SIMPSON, 1957). The tunnel system (a) is infilled (b) following a change in the type of sediment being deposited (bed-junction preservation). Slight degradation of the surface (c) removes the proximal shafts before further sediment, of a different type, accumulates (concealed bed-junction preservation). Renewed degradation of the surface winnows away the sediment, leaving the mucus-lined infilled tunnels as burial preservation (d). (6) Preservation pattern of Arenicolites curvatus (after GOLDRING, 1962). The sedimentary surface with the open U-tubes (a) has been degraded (b), the mucuscemented tube fragments accumulating in an intraformational conglomerate; sediment has filled the tubes in (b). (7) Movement pattern of Diplocration yoyo (after GOLDRING, 1962). In the Upper Devonian Baggy Beds of north Devon, this trace occurs in the various types shown in (F),where all have been truncated to a common erosion surface. It is considered that repeated phases of erosion and sedimentation led to the development of the various types. Stage (A), development of burrow ( I ) . With degradation of the surface, this tube migrates downwards, and at intervals, new tubes (2 and 3) are constructed ( E and C ) . Sedimentation follows (Dand E) but some of the tubes are abandoned. Stage ( F ) , all tubes are abandoned and erosion reduces them to a common base.
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merates. It is not possible to determine how much of the tubes were eroded, though there are fragments up to 5 cm in length. None of the body fossils (thick-shelled pelecypods and gastropods) occur in their growth position. It is interesting to attempt a determination of what the sedimentary surface must have often been like in Baggy times. From a freak preservation at one locality, it can be deduced that the sedimentary surface locally carried a prolific number of DFlocraterion. Judging from the large number of specimens of Lingula, frequently wellpreserved, lying on bedding surfaces elsewhere in the Baggy Beds, other sedimentary surfaces must often have been extensively burrowed by this brachiopod, though not to the depth to which Diplocraterion penetrated. There were probably also banks of the thick-shelled pelecypod Dolabra (Arcacea), none of which have been preserved, the specimens occurring only in cross-bedded sandstone units. If local areas were at times, quite highly populated with surface-dwelling and burrowing organisms, this would indicate (REINECK, 1958), so far as these areas are concerned, periods of only slight sedimentation and erosion. Such areas if they existed at the present day would likely be quite readily observable (e.g., tidal flats) in contrast with areas, such as channels where rapid changes may occur. Scarcely any surface that could represent such an area has been preserved.
AN INTERMEDIATE ASSEMBLAGE
The Baggy Beds and the basin facies of the Blue Lias, or the Lower Jurassic of Wiirttemberg, may be considered as the two extremes for the preservation of sedimentary surfaces. What might be considered as intermediate members, are sections of the Carboniferous of northern England and southern Scotland. There, although there is a good deal of evidence of penecontemporaneous erosion, nevertheless sedimentary surfaces with fossils in their growth positions, and surface-made traces, are not infrequently preserved. The upper part of the Carboniferous limestone and the succeeding Yoredales (Visean to Namurian), together with their Scottish equivalents, comprise repetitive successions of limestone, marine and non-marine shale, sandstone and occasional coals. In palaeoecological studies on the marine shales, CRAIG (1952, 1 9 5 4 , 1956) and FERGUSON (1962) have shown that, occasionally, surface-living organisms such as Productus concinnus and pectiniform pelecypods, and more frequently burrowers such as Lingula and Edrnondia, occur in their position of growth. However, sedimentary scour structures and the occurrence of concentrations of disarticulated and fragmentary shells, show that the sedimentary surface was always likely to be swept by currents. The trace-fossils associated with the series are not well documented, and have often been described merely as “fucoids”. DONALDSON and SIMPSON (1962) have described an association from the Visean (D) limestones of north Lancashire. Besides numerous surface-made, exogenic trails, one bed is covered by conical mounds of castings, Cliornatichnus, above vertical burrows, pushed to the surface by an unknown organism. Judging by the density of the castings (about
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30/m2), they must have been formed on a sedimentary surface of some duration. They were then rapidly covered, and no reworking took place. Spirophyton, Cock’s tail, markings may be connected with the castings, and there are also endogenic burrows, some attributable to Teichichnus and Chonrirites. Cauda-galli markings are common elsewhere in the series, and probably represent the central laminated trace of a sediment-eating organism, migrating through the sediment close to the sea-floor. A Corophium-like burrow with thick walls, near to Tisoa siphonalis MARCEL DE SERRES (BATHER1924), which extends about 5 cm into shale, is common at Earlsferry, Fife, in the Carboniferous Limestone Series. Footprints have been recorded from the Calciferous sandstone (SMITH,1891). In general, the faunal, algal and lithological association indicate a fairly shallow-water sedimentary environment for those parts of the cyclothems considered as marine.
CONCLUSIONS
These analyses, albeit brief, of trace-fossil assemblages formed in no great depth of water, lead to three main conclusions. Firstly, two distinct types of assemblages are represented, possibly the end members of a continuous series, (1) with surface or near-surface made traces, as well as traces made at greater depths, and (2) with only traces made some depth below the sedimen tary surface. Secondly, on the one hand, the frequent occurrence of abundant surface-made traces, or those made very close to the sedimentary surface, indicate fairly continuous sedimentation, and the preservation of nearly every sedimentary surface. On the other hand, the complete absence of these trace types, associated with frequent evidence of erosion, is testimony of discontinuous sedimentation. and the eventual preservation of only a fraction of the actual sediment deposited. REINECK (1960) has shown that only 1/lO,OOO-I/lOO,OOO part of the sediment deposited i n recent shallow water sediments in tidal seas, is actually fossilised. Similar conditions must have existed during the deposition of much of the Baggy Beds. Thirdly, the adjustments of certain organisms attempting to maintain a constant depth below the sedimentary surface, can be used to estimate the amount of sedimentation or erosion that has taken place. Also, the mode of preservation of others is an indication of this. REINECK (1 958) has shown the effect of slow and fast sedimentation and erosion on Mya, Nereis and other organisms in recent sediments. Vertical repetition of Asteriacites and retrusive Diylocraterion give an indicatioii of the amount of sedimentation. These and other examples are shown diagrammatically in the accompanying text-figure. The sedimentological causes for the division are probably complex. To permit continuous sedimentation with only limited penecontemporaneous erosion, the environment must have been under conditions leading t o continuous aggradation. If sediment accumulated beyond a certain rate, colonisation by burrowing organisms would be
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impossible, and most existing organisms would attempt t o migrate elsewhere (REINECK, 1958; MIDDLEMISS, 1962). Discontinuous sedimentation at the present day, is particularly well developed in the intertidal zone of continental shelves, and is a typical feature of prodelta sediments.
SUMMARY
Two distinct trace-fossil assemblages from shallow-water marine sediments are recognised: (a) with traces made on, or close to the sedimentary surface as well as those penetrating deeper, and (h) with only traces made well below the sedimentary surface. Whereas there is evidence for only limited penecontemporaneous erosion in sediments associated with the former type, there is evidence of considerable and frequent penecontemporaneous erosion associated with the latter. The amount of sedimentation and erosion can be estimated from the behaviour pattern and mode of preservation of certain trace-fossils.
REFERENCES
BATHER, F. A,, 1924. Tisoa siphonalis MARCEL DE SERRES, a supposed Liassic annelid. Naturalist, 1925 : 7-10. CRAIG,G. Y., 1952. A comparative study of the ecology and palaeoecology of Lingula. Trans. Edinbrrrgh Geol. SOC.,15 : 110-120. CRAIG,G . Y , 1954. The palaeoecology of the Top Hosie Shale (Lower Carboniferous) at a locality near Kilsyth Quart. J . Geol. SOC.London, 110 : 103-1 19. CRAIG,G. Y.,1956. The mode of life of certain Carboniferous animals from the west Kirkton Quarry, near Bathgate. Trans. Eilinbirrgh Gcol. Soc., 16 : 272-219. DONALDSON, D. and SIMPSON, S., 1962. Choniatichtius. a new ichnogenus, and other trace-fossils of Wegber Quariy. Liverpool Manrhester Geol. J., 3 : 73-81. FEROUSON, L., 1962. The palaeoecology of a Lower Carboniferous marine transgression. J. Paleontol., 36 : 1090-1101. GOLDRING, R., 1962. The trace fossils of the Baggy Beds (Upper Devonian) of North Devon, England. Paliiontol. Z., 36 : 232-257. HALLAM, A., 1960. A sedimentary and faunal study of the Blue Lias of Dorset and Glamorgan. Phil. Trans. Roy. SOC.London, Ser. B, 243 : 1 4 . MIDDLEMISS, F. A,, 1962. Vermiform burrows and rate of sedimentation in the Lower Greensand. Geol. Mqy.. 99 : 3 3 4 0 . REINECK,H. E., 1958. Wuhlbau-Gefiige in Abhangigkeit von Sediment-Umlagerungen. Senckcviber,niana Lethaca, 39 : 1-23, 54-56. REINECK, H. E.. 1960 Uber Zeitliicken in rezenten Flachsee-Sedimenten. Geol. Rundschair,49 : 149-161. SCHAFER, W., 1956. Wirkungen der Benthos-Organismen auf den jungen Schichtverband. Srtickenbet-yiatia Lethaea, 37 : 183-263. SEILACHER, A., 1953. Studien zur Palichnologie. 11. Die fossilen Ruhespuren (Cubichnia). Neites Jahrb. Geol. Palaontol. Abhandl., 98 : 87-124. SEILACHER, A., 1954. Die geologische Bedeutung fossiler Lebensspuren. Z. Derrt. Grol. Ges., 105 : 2 14-227. SEILACHER, A,, 1955. Spuren und Fazies irn Unterkambrium. In: 0. SCHINDEWOLF und A. SEILACHER, BeitrLige zur Kenntnis des Kambriirrns in der Salt Range (Pakistan) - A k a d Wiss. Lit. Mainz Abhandl. Math. Nut. KI., 1955 : 313-399.
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SEILACHER, A., 1958. Zur okologischen Charakteristik von Flysch und Molasse. Eclogae Ceol. Helv., 51 : 1062-1078. SIMPSON, S., 1957. On the trace-fossil Clionrlrites. Quart. J . Geol. SOC.London, 112 : 475-500. SMITH,J., 1891. Note on the occurrence of footprints in the calciferous sandstone between West Kilbride and Fairlie. Trans. Geol. SOC.GlaTyow, 9 : 201-203.