Anatomy of a thinly bedded distal turbidite: Insights from a Proterozoic fan delta system, Simla Basin, Western Lesser Himalaya, India

Turbidites have gained a lot of sedimen¬tological attention during the last decade. However, their relationship to fan-delta systems still deserves further attention. This study addresses an example of thinly bedded... [ view full abstract ]

Turbidites have gained a lot of sedimen¬tological attention during the last decade. However, their relationship to fan-delta systems still deserves further attention. This study addresses an example of thinly bedded turbidites from a prodeltaic sub-environment of a Proterozoic fan-delta system of the Chhaosa Formation, Simla Group, Lesser Himalaya exposed around Rahana village. Detailed process-based facies and paleoenvironmental analysis of the study area have led to identification of more than 1300 m thick coarsening-upwards fan-deltaic successions com¬posed of thinly bedded turbidites overlain by delta slope deposits. Thinly bedded turbidites developed by resedimentation of delta-front sands and slumping of upper pro-delta muds. Erosional features are common at the base of turbidite beds and still more widespread at the top.
The turbiditic sequence is mainly characterized by seven subdivisions (T1-T7): graded bedding with erosional base (T1), thin horizontal laminae (T2), low-amplitude climbing ripples (T3), thin cross-stratified ripple laminae (T4), thin convolute laminae (T5), laminated mud (T6), graded mud (T7). T1 represents the lowermost subdivision of the turbidites where as T6 and T7 constitutes the topmost divisions.
The proposed sequence is analogous to the Bouma structural scheme for sandy turbidites and manly characterizes Bouma's Ta, Tb, Tc, and Td divisions. The repetition of partial sequences characterizes different parts of the slope type fan-delta environment, and represents deposition from different stages of evolution of a large, muddy, turbidity flow developed during normal regression at the stage of stable or marginally rising sea level.

Keywords: Turbidites, prodelta, fan-delta, Proterozoic, Simla Basin, Bouma sequence.