Sequence Of Sedimentary Structures Research Articles

The sequence of sedimentary structures in turbidites, with special reference to dunes

Synopsis The relationship between bed form, grain-size of load, and flow power known from equilibrium-state, open-channel experiments, is revised for turbidity currents, whose power is time-dependent with respect to a point in their path. The revised relationship suggests that cross-bedding, indicative of dune beds, would occur in turbidite sandstones only if the parent turbidity currents had been furnished with sufficiently coarse-grained sediment. Otherwise, the turbidite sandstones are expected to show parallel laminations directly succeeded upward by cross-lamination. The available field evidence is consistent with these conclusions, and some previous difficulties regarding dune structures in turbidites appear now to be resolved.

Scour and fill processes in a deep river hole, Ohio river, Louisville, Kentucky

ABSTRACT A deep hole produced in the Ohio River by suction dredging was measured over a period of two years before and after high water periods. The hole did not fill, but scoured slightly deeper after each high water and the walls remained almost vertical. A vortex or circular eddy developed in the surface flow during high water and sediment was removed from the hole and deposited between the hole and the bank. The movement of water and sediment in a model hole in a flume indicated that a vortex mechanism involving a balance of scour and fill is keeping the hole free of sediment during each period of flood. The sequence of sedimentary structures for the model hole included foreset type beds and dune bedding migrating both upstream and downstream. The final structures preserved depend on the relative lengths of the phases of scour and fill and the maximum mean velocity of the stream.

Comparison of Recent and Ancient Coarse-Grained Point Bars: ABSTRACT

Sequences of sedimentary structures in modern point-bar deposits of the Amite River in east Baton Rouge Parish, Louisiana, are analogous to features observed End_Page 731------------------------------ in Eocene Simsboro deposits in Milam County, Texas. Annual rainfall of approximately 60 in., channel pattern slightly to highly meandering, average stream gradient of about 5 ft/mi, and bank stabilization by dense vegetation are major parameters controlling Amite deposition of coarse sand and pebble-gravel sediment. Stratification types are related directly to specific depositional features, and include (from thalweg to overbank): thalweg (large-scale cross-stratification), lower point bar (trough cross-stratification, avalanche beds), chute bar (parallel laminae, avalanche beds, trough fill), chute fill (parallel inclined laminae, climbing ripples), and overbank (parallel inclined laminae, mud drape, avalanche beds). Fundamental differences between point bars of streams transporting coarse-grained bed load and streams having fine-grained bed load are: muddy floodplain deposits are associated only with fine-grained bed-load streams; upper point-bar sediments with ripple cross-stratification and parallel inclined laminae occur only in fine-grained fluvial deposits; chute-front avalanche beds are common in coarse-grained fluvial deposits but are not found in fine-grained fluvial deposits. Coarse-grained fluvial deposits do not show the upward decrease in grain size that is reported to characterize fluvial sediment. The Simsboro consists mainly of thalweg, lower point-bar, and chute-bar deposits; chute-fill and overbank deposits are preserved only in the last depositional sequence. End_of_Article - Last_Page 732------------

FABRIC OF A MIDDLE ORDOVICIAN LIMESTONE AT COLBORNE, ONTARIO

The varieties of limestone in the Middle Ordovician Cobourg Formation at Colborne, Ontario occur as follows: sparse biomicrite, 49%; fossiliferous micrite, 26%; biosparite, 23%; micrite, 2%; and packed biomicrite, trace.Some individual beds display a sequence of sedimentary structures composed of graded bedding at the base, followed by a zone of lamination, and structureless micrite at the top. Most disarticulated brachiopod and ostracod shells are convex upward in biosparite and convex downward in biomicrite and fossiliferous micrite. The percentage of brachiopod shells decreases towards the top of the quarry and changes laterally. Mechanical orientation patterns of elongate fossils show two dominant trends, NE–SW and NW–SE. Factor analysis suggests that the association of brachiopod shells and pelmatozoan fragments was controlled by a common environmental influence, that the bryozoa and ostracod remains were transported together with reworked quartz grains, whereas the distribution of trilobite fragments was controlled by another condition.The carbonate sedimentation occurred in a marine environment of generally low turbulence. Stronger currents were introduced sporadically to form graded and non-graded biosparites.

Use of Sedimentary Structures in Determination of Depositional Environments, Mesaverde Formation, Williams Fork Mountains, Colorado

Rock-stratigraphic units composing the Mesaverde Formation of northwestern Colorado were deposited in offbeach, beach, lagoon, swamp, and floodplain environments of deposition. The vertical distribution of these rock-stratigraphic units reflects the lateral distribution of environments at the time of deposition. In a similar manner, the vertical distribution of sedimentary structures within the rock-stratigraphic units reflects the lateral distribution of processes at the time of deposition. Therefore, an understanding of modern processes and the distribution of their resultant sedimentary structures will permit prediction, through the principle of uniformitarianism, of the vertical distribution of stratification within the depositional products of ancient environments. > It is possible to subdivide the products of the various nearshore marine and nonmarine deposits into a series of subenvironments based on types of stratification. In a regressive suite of rocks, beginning with the offbeach environment, the rocks are characterized by parallel laminae and low-angle cross-laminae reflecting deposition from suspension and from low-energy traction currents. Landward, through the beach environment, varying processes and energy levels result in a sequence of sedimentary structures characteristic of the offshore-beach, submarine-bar, and foreshore-beach subenvironments. The beach zone may be overlain by rocks of the lagoonal, swamp, or floodplain environments. The lagoonal environment comprises sedimentary structures indicative of processes in tidal-delta, ti al-channel, lagoon-pond, and tidal-flat subenvironments. The swamp environment is characterized predominantly by lithology--coal and carbonaceous sediments--rather than by stratification. The floodplain environment comprises stratification characteristic of the channel and interchannel subenvironments.

Fluvial Processes as Interpreted from Ancient and Recent Fluvial Deposits: ABSTRACT

Detailed outcrop studies of fluvial deposits in the Missourian of Oklahoma and the Atokan of the Arkoma basin have revealed a systematic vertical variation in grain size, sedimentary structures, bedding characteristics, and morphology of sedimentary units. The frequency with which this ideal sequence is developed suggests a common process in the deposition of many of these fluvial sandstones. The ideal vertical sequence is as follows: (1) a lowermost festoon cross-bedded zone related to sand wave and (or) dune transport; (2) a parallel laminated zone deposited in low amplitude sand waves or in a plane bed; (3) a fine-grained symmetrical ripple zone containing sediment formerly transported in suspension; and (4) a laminated clay and fine-grained sand zone deposited f om suspension (commonly outside of the fluvial channel). This sequence has been found in both Recent and ancient fluvial deposits. Flume and river studies have demonstrated that specific sedimentary structures are directly related to sediment transport and the dynamics of open channel flow. Sedimentary structures in the ancient sandstones of the study area were related to bedforms described from Recent channels. Furthermore, additional information on the morphology of the fluvial channels was obtained from an analysis of the thickness and distribution of specific zones. The grain-size distribution of the fluvial sands of the study area suggests an upward decrease in energy. Both the mean and maximum grain size decrease upward, and the sediment is progressively more poorly sorted upward. These changes are directly related to variation in the type and size of the sedimentary structures. Discontinuities resulting from successive floods were identified by abrupt changes in grain size and (or) in the sequence of sedimentary structures. The ideal vertical sequence, as given is commonly encountered in the field. This suggests some common underlying cause. This study illustrates that use of the flow regime concept, together with an understanding of river hydraulics and channel behavior, can provide a better insight into fluvial sedimentation. End_of_Article - Last_Page 550------------