Age Revision of the Mariakani Formation of the Upper Duruma Group in South Lamu Basin of Kenya Based on Preliminary Calcareous Nannofossils Biostratigraphy

Abstract

This paper provides new biostratigraphic evidence of the onlapping of the Jurassic and Cretaceous strata above older formations of the Duruma Series in the southern Lamu Basin. A calcareous nannofossils biostratigraphy study was undertaken on three samples consisting of sandstone and shale units outcropping in an area and mapped initially belonging to the non-marine Mariakani Formation of the Karoo-Duruma Group in the southeast coastal Kenya. This was done in an attempt to upgrade and construct a detailed Meso-Cenozoic chrono – and biostratigraphic framework for this area that will be essential for regional and global correlations, paleogeographic reconstructions and exploration for hydrocarbon, water and other mineral resources. As a result, a total of nineteen moderately-to-well-preserved taxa characterizing the Pseudoconus enigma (NJ11) Zone of the Boreal or Watznaueria barnasae (NJT11) Zone of the Tethyan zonations were identified. The assemblage of Sample №61 is dominated by such species as Watznaueria barnesiae, W. britannica, Schizosphaerella punctulata, Lotharingius cf. L. contractus, Lotharingius sigillatus, ?Discorhabdus striatus and Watznaueria manivitiae. Both nannofossil zones correlate this layer with the lowermost Bathonian Zigzag Ammonite Zone.

The overlying light grey-blue shale unit – samples №60 and 66 have an (?) Lower Cretaceous (Albian) age based on the presence of two zonal species – ?Axopodorhabdus albianus and Eiffelithus turriseiffelii that characterize the lower Eiffelithus turriseiffelii (BC27/NC10/CC9) Zone which makes them a correlative analogue of the fossiliferous Walu shales widely developed in the northern sector of the Post-Karoo Lamu Basin.

The studied sample №61 is stratigraphically younger than the upper shaly units of the Lower Member of the Kambe Formation from the Mwache River sections located some 25 km to the NE. This biostratigraphical evidence is the first recognition proving the presence of Early Bathonian strata in the Indo-Malgach Province since only Late Bajocian and Middle Bathonian ammonites have been described from this area. The examined samples, especially the Mid-Jurassic share most of the zonal species determined in the corresponding stratotypes confirming marine connections with the Tethyan and Boreal realms. Samples 60 and 66 correspond to the Eiffelithus turriseiffelii Zone (CC9/NC10), and correlate with the Walu shales developed in Northern Post-Karoo Lamu basin sections and the basal part of the Lindi Formation of Southern Tanzania. The correlations with the Bathonian and Albian stages stratotype sections and Ammonite zonations are also provided.

This study demonstrates the complex geological structure of the area with evidence of multiple transgressional-regressional episodes during the Jurassic and Cretaceous times. It also shows the need for a complete revision of the age for all mapped rock formations developed in the Karoo-Duruma and Post-Karoo Lamu basins. These results also prove that the Late Mid Jurassic and Mid-Cretaceous marine transgressions progressed much farther to the west than originally mapped. The data renew and continue the discussion of how far west did the Jurassic marine sequences overlap the Karoo-Duruma Group deposits that was first suggested by some researchers in the early 20th century.