Lower Karoo Sedimentation in Moatize Basin, Mozambique: Analogue for Understanding the Deep Rift Sequence in Offshore Rovuma Basin
Authors: Pendkar Narender , Rubino Jean-Loup , Jean-Baptiste Baby Guillaume , Sia Simon Seak Lung , Prasetyotomo Wahyu Singgih 
Published: March 2015

This paper reports observations carried out on Carboniferous to Permian sedimentary rocks in Moatize Basin in Tete Province. The outcrops of Vuzi, Moatize and Matinde Formations ranging in age from Late Carboniferous to Late Permian were logged and sampled along river sections. The Vuzi Formation is made up of diamictites with thin bedded sandstone intercalations, laminated shale and localized hemispheroidal stromatolitic limestone which were deposited in glacio-fluvial to lacustrine depositional systems. The stromatolitic and laminated limestone was formed during a deglaciation episode on the lacustrine margin. The younger outcrops of Moatize Formation are characterized by thick stacked braided and meandering fluvial, and lacustrine deposits with intervening coal seams. The lacustrine deposits are characterized by fissile shales with thin, laterally persistent fine grained sandstones and siltstones. The lacustrine depositional systems range from shallow, with localized deposits of several meters of laminated grey silty shales within the confines of fluvial deposits, to deep, with a thickness of tens of meters having dark grey shale associated with rhythmic turbidites.

Introduction

The spate of hydrocarbon discoveries in Rovuma Basin, Mozambique during the recent years (Fruhauf, 2014) have paved way for an outline of petroleum systems. While the role of Tertiary and Mesozoic geological elements are well understood (Salman and Abdula, 1995), the stratigraphic and tectonic framework of deeper Karoo sedimentary sequences has not been completely deciphered (Figure 1). The regional sequence stratigraphic analysis studies carried out by Sapri et al. (2013) in southern part of Rovuma basin has established up to five sequences from Triassic to the Present. With absence of well data, the characteristics of the deeper Karoo sedimentaries could not be recognized with confidence based on seismic data alone. The objectives of the study are to use outcrops as analogues for understanding sedimentological and source rock characteristics for the deeper part of rifted sedimentary sequence within the offshore Rovuma Basin.

Moatize Basin

The Karoo sedimentation in various south and central African basins has been extensively documented (Cairncross, 2001; Catuneanu et al., 2005). For this study, Moatize Basin was chosen as it offers excellent outcrops of sedimentary rocks deposited in a rift sequence ranging in age from Late Carboniferous to Late Permian, making it an ideal analogue for understanding sedimentological and source rock characteristics for the deeper part of rifted sedimentary sequence within the offshore basins like Rovuma, the site for recent prolific hydrocarbon discoveries on the eastern margin of Africa. In addition to the coal bearing outcrops of the Permian encountered along the Muaradzi and Moatize river sections, the Vuzi Formation, the equivalent of the Dwyka Tillite was also examined along the Murongozi river section (GTKa, 2006).

The geology of the Tete Province is structurally dictated by the regional Zambezi Rift, which trends E-W from the Zambia-Zimbabwe border, then swings SE to S-SE through Tete. The rift was formed in Proterozoic basement rocks, developing into a zone of active extensional tectonism and sedimentary deposition. The Proterozoic basement comprises of igneous, crystalline rocks, schists and metasediments (GTKa 2006). The geological structure in the Moatize Basin is dominated by a series of high angle faults, which divide the basin (Vasconcelos, 2000).