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Savanna Journal of Basic and Applied Sciences 3(2):124-138
Sequence Stratigraphy and Sea Level changes in Relation to Evolution of Anambra Basin, Southeastern Nigeria
Ola-Buraimo A.O and Ehinola O.A.

ABSTRACT
Biostratigraphic sequence stratigraphy technique was used to predict the stacking pattern and geologic events towards understanding sea level changes as precursor for the evolution of Anambra Basin, Nigeria. Palynology analysis data and lithologic description of Nzam-1 well comprising oldest Asu River Group to youngest Ogwashi/Asaba Formation were used. The method involved laboratory preparation of palynology slides and detailed identification of pollen, spores, dinoflagellates, algae, fungal spore, microforaminiferal wall linings and other stratigraphically important forms present. The method entails total abundance and diversity of palynomorphs and cognizance of two types of dinoflagellates such as ganyaulacaceans and peridinaceans. The preponderance of ganyaulacaceans in addition to high abundance and diversity of palynomorphs defines a stratigraphic Condensed Section (CS) within which is Maximum Flooding Surface (MFS), characterizes Highstand Systems Tract (HST). On the other hand, intervals characterized by barren to poor recovery of palynomorphs suggest lowest position of sea level, equivalent to Lowstand Systems Tract (LST) at the base of which is a Sequence Boundary (SB). The rising sea level is the Transgressive Systems Tract (TST) characterized by relative increase in palynomorph abundance, diversity and higher occurrence of peridinacean dinoflagellate cysts compared to LST. Eight transgressive cycles (TCs) and 5 major unconformity surfaces synonymous to regressive surface were identified at different geologic time as a result of eustatic changes in sea levels. The deepest part is characterized by TST/HST suggestive of marine incursion during Albian due to splitting of the Gondwana land; responsible for the deposition of Asu River Group. The second MFS was located at 3125 m, dated Upper Cenomanian, responsible for the accumulation of Eze Aku Formation. A regressive (LST) was responsible for the deposition of Awgu Formation in the Coniacian period. The third transgressive cycle occurred during the Early Campanian, responsible for the deposition of Nkporo Formation; MFS placed at 2411 m, dated Upper Campanian within the formation. The lower part of Mamu Formation was deposited under the influence of LST, dated Lower Maastrichtian; SB placed at 2057 m. The Middle Maastrichtian was marked by the forth TC; deposited upper Mamu Formation. However, the lower boundary of Nsukka Formation was marked by LST placed at 1234 m, dated Lowermost Upper Maastrichtian within Nsuklka Formation. The sixth marine transgression deposited Imo Formation, followed by a regressive phase (LST) in the Upper Late Paleocene. The seventh marine transgression occurred during Middle Eocene, led to deposition of Ameki Formation while the eighth marine transgression was in the Upper Miocene-Pliocene period, responsible for deposition of Ogwashi/Asaba Formation