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Das, Subhasish
- Geometrical Analysis of Two Dimensional and Three Dimensional Oscillation Ripples
Authors
1 Department of Geology and Geophysics IIT Kharagpur 721302, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 32, No 6 (1988), Pagination: 447-460Abstract
Analysis of ripple geometries using known tabulated laboratory and field data shows that the geometry can be of three types, i.e., sand ripples, light sediment ripples, and oscillatory bed ripples, reflecting different effects of the fluid shear stress and fluid accleration on the bed sediments. It is shown that when the bed shear stress or the amplitude of the water particle relative to the sediment size is small, the resulting vortices are more regular with low turbulence intensity and the generated ripples are two dimensional. However when the bed shear stress is large, the resulting vortices are very irregular with high turbulence intensity and the generated ripples are three dimensional in character.
- Fluidized Sediment Movements and Deposition - A case study from Bisrampur, Singhbhum Dt., Eastern India
Authors
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur-721 302, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 45, No 3 (1995), Pagination: 301-310Abstract
Critical studies of a part of Singhbhum basin around Bisrampur, Singhbhurn district, Bihar, show that the metasediments (considered to belong to the Dhanjori Group) lie unconformably above the Singhbhurn Granite. However a few tongues of the granite that occur within the metasediments have been explained as thrust slices of the basement within the cover rocks. Though three phases of deformation have been recorded here, and the general outcropgattern has been largely controlled by the second phase of deformation, there has not been any marked change in the spatial disposition of the coarser clastics. The shape of pebbles within the conglomerate, though often triaxial, cannot be correlated with the deformational history. This calts for original triaxiality of the pebbles. The high proportion of clastmatrix indicates that they were deposited under a two phase flow mechanism: a gravity mass flow aided by a high turbulent flow. Relatively high amounts of solid material in the fluid with variable particle size can create such a turbulent motion and the resultant scattered dispersion possibty accounts for the randomness in pebbleorientation. Accumulation of huge coarse clastics is, however, attributed to gravity mass flow. The general lack of turbidite structures in these litho-units can be attributed to flow inhomogeneity, flow velocity, nature of depositional substrate, relative significance of head-versus-body spilling from turbidity current. Presence of primary structures in arenaceous rocks and occurrence of these rocks as isolated bodies point to vigorous reworking and redeposition of the bottom sediments after cessation of the main flow. The huge pile of the rnicaceous phyllites probably represents relatively rapid deposition of silt grains from a highly concentrated mud-dominated turbidity current or very fluid silty debris flow deposited during the waning phase of turbidity flow. The entire load was deposited possibly over a shallow shelf edge. Retention of the original grain fabrics and the sedimentary structures in these metasediments, even though they are very close to a regionally developed shear zone, points to the existence of focally developed, sheltered low stress regimes provided by the Granite basement.Keywords
Sedimentary Structures, Sedimentation, Dhanjori Group, Singhbhum, Bihar.- Depositional Framework of the Sandy Mid-Fan Complexes of the Proterozoic Chaibasa Formation, E.Singhbhum, Bihar
Authors
1 Department of Geology and Geophysics, Indian Institute of Technology, Kharagpur-721 302, W.Bengal, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 50, No 5 (1997), Pagination: 541-557Abstract
The Proterozoic Chaibasa Formation belonging to the Singhbhum Group was deposited in small overlapping submarine fans along the eastern cratonic margin of an unstable foreland trough during the early phase of the Singhbhum Orogeny. The following features suggest deposition within the mid-fan complex: (I) a relatively high proportion of coarse, massive psammitic layers with small proportion of pelitic layers and classic graded sandstones; (2) the presence of shallow sand-filled channels; (3) the frequent associations of olistostromes; (4) association of thick coarse sandstone with thin layers of fine sandstone separated by thick shale units; (5) the presence of both positive and negative cycles in the stratigraphic record.
Sandstone bodies characteristic of the unchanneled fan lobes, represent deposition associated with basinward progradation of the lobe complex. Accumulation of a thick siltstone on top of the coarse sandstone facies during the active progradational phase is related to a channel cutoff in the upfan region. In contrast, the sandstone bodies present in the channel show an overall graded nature, lack thick siltstone horizon, contain erosional structures, and a higher percentage of positive cycles. The development of the submarine fan was related to external and internal factors, that is, the flow velocity, the rate of sediment supply coupled with channel migration, frequency of major storms or earthquakes to initiate submarine gravity flows. This development of alternating graded sandstone units and siltstone blankets may be a more useful criterion (than sedimentary structures) for the recognition of ancient mid-fan lobe sequences.