Tb or not Tb: Banding in Turbidite Sandstones

Submarine gravity flows include a range of flow types from turbidity current to debris flow, and hybrid flows that are transitional between the two. Their associated deposits range from clean to mud-rich sandstones, and... [ view full abstract ]

Submarine gravity flows include a range of flow types from turbidity current to debris flow, and hybrid flows that are transitional between the two. Their associated deposits range from clean to mud-rich sandstones, and composite deposits of both mud-rich and clean sandstone. This contribution examines banding, an enigmatic hybrid facies comprising mud-rich bands intercalated within clean turbidite sandstones. A suite of core and outcrop data is presented from two exhumed ancient systems and a subsurface producing oilfield: the Karoo Basin, South Africa; the Neuquén Basin, Argentina, and; the Magnus Oilfield, North Sea. These data show that banding occurs in proximal localities forming complex heterolithic bedforms, towards the tops of beds overlying structureless sandstones, and commonly intercalated with Bouma Tb clean sandstone. Mud-rich bands are strongly associated with erosion into muddy substrate: subtending directly from erosion surfaces, and comprised largely of mud clasts. We interpret banding as a product of transitional flow behaviour due to the presence of cohesive mud from erosion, coupled with deposition of clean sands by traction carpets. The nature of the eroded mud (clay type and degree of consolidation) and the rate of aggradation onto the bed dictate the style of banding. An improved understanding of banding enables it to be used as a tool in the field and subsurface to recognize paleoenvironments, and interpret the rich diversity of flow processes that supply sediment to deep-water systems.