Sedimentology and paleoenvironments of a new fossiliferous late Miocene-Pliocene sedimentary succession in the Rukwa Rift Basin, Tanzania

Abstract

This paper presents a detailed sedimentologic investigation of a newly identified, fossiliferous Late Neogene sedimentary succession in the Rukwa Rift Basin, southwestern Tanzania. This synrift deposit is a rare and significant new example of a fossiliferous succession of this age in the Western Branch of East Africa Rift System. The unit, informally termed the lower Lake Beds succession, is late Miocene to Pliocene in age based on crosscutting relationships, preliminary biostratigraphy, and U-Pb geochronology. An angular unconformity separates the lower Lake Beds from underlying Cretaceous and Oligocene strata. Deposition was controlled by rapid generation of accommodation space and increased sediment supply associated with late Cenozoic tectonic reactivation of the Rukwa Rift and synchronous initiation of the Rungwe Volcanic Centre. The lower Lake Beds, which have thus far only been identified in three localities throughout the Rukwa Rift Basin, are characterized by two discrete lithologic members (herein A and B). The lower Member A is a volcanic-rich succession composed mostly of devitrified volcanic tuffs, and volcaniclastic mudstones and sandstones with minor conglomerates. The upper Member B is a siliciclasticdominated succession of conglomerates, sandstones, mudstones and minor volcanic tuffs. Detailed facies analysis of the lower Lake Beds reveals various distinctive depositional environments that can be grouped into three categories: 1) alluvial fan; 2) fluvial channel; and 3) flood basin environments, characterized by volcanoclastic-filled lakes and ponds, abandoned channel-fills and pedogenically modified floodplains. Member A represents a shallow lacustrine setting filled by tuffaceous sediments, which grade up into a system of alluvial fans and high-energy, proximal gravel-bed braided rivers. An unconformity marks the contact between the two members. Member B shows an upward transition from a high-energy, gravel-bed braided river system to a sandy braided river system with increasingly abundant floodplain deposits and well-developed paleosols. Vertebrate fossils are sparse in member A, but common in member B, preserved both within pedogenic soil horizons and as isolated elements and microsites within fluvial channel facies associations. Faunal remains include fishes, turtles and crocodylians, along with wellpreserved mammal cranial and post-cranial remains. In addition, freshwater gastropod shells are locally present in member A and continental trace fossils, including abundant fossilized termite nests, are present in both members.

Structural geometry and evolution of the Rukwa Rift Basin, Tanzania: Implications for helium potential

“…Within these faults, the synrift strata show sagging in the hanging wall synformal structure, most likely due to differential compaction and loading, that is, variation of thickening and thinning affecting both the Oligocene Nsungwe Formation and the lower Lake Beds sequences (Late Miocene‐Pliocene Malangali Formation) (e.g. Hilbert‐Wolf et al, 2017; Mtelela et al, 2017; Figure 9c). The synkinematic wedges and onlaps indicate that these folds initiated in the Oligocene (c. 25–26 Ma) and deformation continued until the Pleistocene (c. 2.6 Ma?)…”

“…These distinct intervals most likely record the Karoo Supergroup deposition between late Carboniferous (Pennsylvanian)-Permian (Cisuralian) and late Permian (Lopingian) respectively with a hiatus that is possibly Guadalupian age (cf. Delvaux, 2001;Hilbert-Wolf et al, 2017;Mtelela et al, 2017;Widlansky et al, 2018; Figure 2; Figure 6).…”

“…The Red Sandstone Group unconformably overlies the Karoo Supergroup and is divided into two formations including the Cretaceous fluviatile Galula Formation, and the Oligocene Nsungwe Formation composed of mainly quartzose and richly fossiliferous fluvial‐lacustrine sandstone sequences (Mtelela et al, 2017; Roberts et al, 2010; Widlansky et al, 2018). Finally, the uppermost strata consist of Late Miocene‐Pleistocene Lake Beds Group which comprises two subdivisions including the Late Miocene‐Pliocene Malangali Formation and the Pleistocene Ilasilo Formation (Hilbert‐Wolf et al, 2017; Mtelela et al, 2017; Figure 2). The uppermost strata contain records of the modern tectonic development of the EARS and associated extensive influence from the Rungwe Volcanic Provinces (RVP) (Ebinger et al, 1989; Lawrence et al, 2021; Mtelela, 2016; Wescott et al, 1991).…”

“…The Karoo Supergroup comprises the Late Carboniferous to Permian units that lie unconformably on Precambrian Basement (Kilembe & Rosendahl, 1992; Morley et al, 1999b; Figure 2). The Red Sandstone Group unconformably overlies the Karoo Supergroup and is divided into two formations including the Cretaceous fluviatile Galula Formation, and the Oligocene Nsungwe Formation composed of mainly quartzose and richly fossiliferous fluvial‐lacustrine sandstone sequences (Mtelela et al, 2017; Roberts et al, 2010; Widlansky et al, 2018). Finally, the uppermost strata consist of Late Miocene‐Pleistocene Lake Beds Group which comprises two subdivisions including the Late Miocene‐Pliocene Malangali Formation and the Pleistocene Ilasilo Formation (Hilbert‐Wolf et al, 2017; Mtelela et al, 2017; Figure 2).…”

Morphological diversification of ampullariid gastropods (Nsungwe Formation, Late Oligocene, Rukwa Rift Basin, Tanzania) is coincident with onset of East African rifting

“…, , , , ); and (4) deposition of the Pliocene to Recent Lake Beds Succession, a complex volcanically influenced fluvial deltaic to lacustrine system with alluvial fan deltas on the basin margins (Hilbert‐Wolf & Roberts ; Mtelela et al . , ; Hilbert‐Wolf et al . ).…”