Origin of conglomerate-hosted bonebeds at the base of the upper Permian Usili Formation, Ruhuhu Basin, Tanzania

A diverse conglomerate-hosted fossil tetrapod assemblage occurs in a ∼4 m-thick interval at the base of the upper Permian Usili Formation in the Ruhuhu Basin, Tanzania. The interval comprises three 0.5–1.5 m-thick erosively based fining-upward sequences. Each begins with a lenticular body of clast to matrix-supported conglomerate capped by a thin bed of massively bedded granule conglomerate and sharply overlain by fine-grained sandstone. Conglomerate intraclasts include abundant spheroidal carbonate nodules with septarian shrinkage cracks, rounded mudstone pebbles, scattered carbonate mud discs and fragments of stromatolitic limestone, all locally-to-regionally sourced from erosion of the underlying Ruhuhu Formation lacustrine and lake-plain deposits. Isolated cranial and postcranial elements of burnetiamorphs, dicynodonts, gorgonopsians, pareiasaurs, and temnospondyls occur scattered within the conglomerate and overlying gritstone beds, becoming abundant in the capping fine-grained sandstone along with silicified tree trunks. Significantly, most bones are preferentially encrusted with hematite, which suggests generally wet soil conditions with seasonal drying, and tree rings show the normal seasonal growth patterns of a humid temperate climate. The basal Usili conglomerates are attributed to a period of tectonic reorientation of the Ruhuhu rift bounding faults resulting in a basin-wide transition from deep lacustrine (upper Ruhuhu Formation) to alluvial plain with an episode of axial river incision and lateral alluvial fan progradation (basal Usili Formation) before subsidence increased to allow floodplain aggradation (lower-to-upper Usili Formation). Taphonomic signatures of most of the bones suggests they were sourced from carcasses lying exposed on terrace surfaces and in alluvial fan channels rather than from reworking of previously fossilized bones.