Depositional Enviroment And Origin Of Deformation Structures In The Upper Cretaceous Facies Of Enugu Area, South Eastern Nigeria

摘要

ABSTRACT : - The Upper Cretaceous facies of Enugu Area are part of the sequence in the Anambra Basin of Southeastern Nigeria. These include the Enugu Shale, Mamu Formation, Ajali Sandstone and Nsukka Formation. The deformation structures within the Upper Cretaceous in Enugu area were dominant to sandstones of the Marnu Formation. The white silty fine sandstone, muddy fine sandstone and heteroliths of this formation show various degrees of deformation which were penecontemporaneous with the deposition of the lithologies. The deformation structures include convolute bedding, load and flame structures, slump structures, gravity faults, water escape structures. Other sedimentary structures in the area are wave ripple lamination, hummocky cross stratification, parallel lamination, current ripples, flaser and lenticular bedding, channeling, water level marks, tabular and lenticular sandstone, mudcrack etc. The vertical, inclined and horizontal burrows of Ophiomorpha, Thalassinoides and Skolithos constitute the biogenic structures. Studies have shown that these deformation structures in the area originated from several processes which include the instability of the over steepened depositional slopes, overloading and unequal loading by sediment which produced both vertical and lateral movement (differential liquefaction and thixiotropic transformation) and rapid sedimentation under gravity. Diagenetic and textural studies indicate that the sandstones are now well compacted and lithified. The presence of these inorganic and biogenic sedimentary structures and the general stratigraphy of the area indicate that the sandstones were deposited in a storm dominated and low wave energy marine shoreline environments.