塔里木盆地晚泥盆世东河塘组河口湾相遗迹化石

河口湾具有特定的环境条件和沉积组成，其遗迹化石具有半咸水沉积的遗迹群落特征。描述和分析塔里木盆地塔中4井区上泥盆统东河塘组河口湾沉积中的遗迹化石和生物扰动构造后，发现3类遗迹组构，其中Ophiomorpha遗迹组构发育在纯净砂岩中，与河口湾潮汐砂坝有关；Skolithos遗迹组构发育在薄层砂岩中，与河口湾砂坪有关；Palaeophycus遗迹组构发育在泥岩中，与河口湾泥坪、砂泥坪有关。

TRACE FOSSILS FROM ESTUARINE FACIES OF DONGHETANG FORMATION (UPPER DEVONIAN), TARIM BASIN

Ancient estuarine deposits have not widely been recognized, mainly because the estuaries consist of a complex set of intergradational subenvironments that display considerably variation, and also because the major environmental factor that influences the distribution of the organism (i.e.variable salinity) is not recorded ostensibly in the sediments. They can be recognized by four features as follows: 1)The spatial distribution and evolution of estuarine subfacies are very important for recognizing the estuary. The genaral nature of the stratigraphic succession produced by an estuary as sea level rises from the fluvial deposits of lowstand to estuarine sediments of transgression and subsequently stabilizes at a highstand. 2)The biogenic structures found in the estuary are individually more characteristic than other depositional environments. The mixture of suspension-feeding sand dwellers and deposit-feeding mud dwellers constitutes a mixed Skolithos-Cruziana ichnofacies. 3)Highly heterogenereous sedimentary structures, bi-to polymodal current orientations indicate that the estuaries are controlled by the interplay between marine processes (waves and tides) and fluvial processes.The ichnology of estuaries displays typical features of brackish water environment: 1)low diversity of ichnotaxa regardless of the abundance of individuals, which may be high; 2)a bias towards morphologically simple structures reflecting simple feeding stratigies; 3)dominance of infaunal forms over epifaunal forms; 4)presence of an impoverished marine assemblage of ichnofauna rather than a mixed freshwater/marine assemblage, and diminished size relative to fully marine counterparts; 5)presence of elements of both the Skolithos and Cruziana ichnofacies.Based on the synthetic analysis on bioturbation index and ichnofabric constituent diagram, three ichnofabrics have been recognized from estuarine facies of Donghetang Formation (Upper Devonian) of Tarim Basin,western China, they are: Ophiomorpha ichnofabric, Skolithos ichnofabric and Palaeophycus ichnofabric.Ophiomorpha ichnofabric is characterized by the widely occurrence of Ophiomorpha as well as the absence of other trace fossils. The trace fossil Ophiomorpha ranges from simple, individual burrows to irregular three-dimensional burrow networks consisting of cylindrical tunnels and shafts that typically bifurcate at acute angles. The pelleted wall lining is unclear in the Donghetang Formation because of the strong diagenetic cementation in the burrows. The burrows are very obvious in core because they are commonly high-lighted by contrasting colors of the burrow fill and the surrounding matrix. The burrow fill is much lighter than the matrix and has different texture and fabric. The burrow fill, greyish white in color, consists of secondary ferroan calcite. The matrix, dark brown in color, is constituted by oil-soaked quartz sands. The bioturbation structures formed by Ophiomorpha tracemaker show a different bioturbated degree. They can be divided into 6 grades (bioturbation index), each grade of bioturbation (or BI)is clearly defined in terms of burrow density, amount of burrow overlap and preserved percent of the original sedimentary structures and represents different petrophysical properties. Bed by bed core analysis and statistics of bioturbation indices of Ophiomorpha ichnofabric indicate that the index 2 is common and amounts to 30% of the measured strata, other indices have low percent, absence of bioturbation constitutes 40% of the measured strata. This pattern of bioturbation index distribution reflects the deposition is dominated by physical processes and resembles that of ichnofabric index from high-energy sandstones described by Drose and Bottjer(1989). The Ophiomorpha ichnofabric is preserved in thick-bedded "clean" quartz sandstone interpreted as estuarine tide-bar sedimentation with low-angle bidirectional cross-bedding and tabular cross-bedding. Similar occurrences of Ophimorpha are reported from many localities (Bromley, 1990; Pollard et al., 1993; Anderson and Droser, 1998). The sandstones have a good textural maturity and compositional maturity.Skolithos ichnofabric is preserved in cm-dm fine sandstone beds interbedded cm units of mudstone beds with small scale parallel lamination, low angle bimodal cross-bedding and clay-draped laminae. The uniform dispersion of Skolithos burrows suggests that the reworking of trace makers to sediments is minimal, the primary sedimentary structures here are well preserved in most sections. Skolithos burrows are deeper than those in normal marine sediments indicating fluctuating water salinity. Deep burrows allow suspension feeders to maitain optimum down-burrow microenvironments in environments of fluctuating water salinity. This ichnofabric can be interpreted as formed in estuarine sandflat environments.Palaeophycus ichnofabric is preserved in mudstone and silty mudstone beds and associated with horizontal bedding, wave bedding, sandstone lens and syneresis cracks.The bioturbation degree is low and represented by low density of Palaeophycus, and Planolites. Syneresis cracks are very common accessory sedimentary structures found in argillaceous beds which display this ichnofabric. They may provide evidence that the depositional environment was subjected to periodical, extremely strong salinity fluctuations. The nature of Palaeophycus ichnofabric, argillaceous sediments and horizontal, deposit-feeding structures suggest that this ichnofabric is related to estuarine mudflat environment.