Deep water fold-thrust belts in Krishna-Godavari basin, East coast of India Mainak Choudhuri 2011

Abstract Rapid Oligocene to Pliocene sedimentation in the Krishna-Godavari basin in the east coast of India has resulted in the development of two distinctly different styles of linked extension- compression systems. In general, toe thrusts are developed in the Krishna basin, while shale bulges develop in the Godavari basin. Structural restoration is attempted to understand the fault kinetics leading to the formation of these deep water fold-thrust belts. Their origin is related with high sedimentation rate in Oligocene and Pliocene, leading to failure along to overpressured shale horizons along Cretaceous-Paleocene top. The triggering cause seems to be over-steepening of the shelf edge, leading to collapse and formation of growth faults, which progress in age from Oligocene to Miocene to Pliocene from south to north. Other factors influencing the formation of fold-thrust belts are the local detachment slope and underlying basement structures.

1.Study area 2.Types – toe thrusts and shale bulges 3.Characteristics 4.Restoration of representative sections 5.Controlling factors 6.Summary Framework

Study area Krishna basin Godavai basin

DWFTB types Two distinct morphologies of linked extension-compression structures 1.Toe thrusts 2.Shale bulges Common characteristics Associated with prolific growth sequences Basinward sloping detachment Failure by gravity gliding Updip extension balanced by downdip compression

Sections Toe thrusts Shale bulges

Toe thrusts 1.Found in the Krishna basin 2.Growth in Pal-Oligo and L. Oligo-E. Mio 3.Steep basal detachment 4.Basinal progression of thrusts # 5.Little control of underlying structures

Shale bulges 1.Found in the Godavari basin 2.Growth in E.-M. Mio and E. Plio 3.Gentle basal detachment 4.Landward progression of bulges 5.Controlled by underlying basement geometry

Restoration of a toe thrust in the Krishna basin Cretaceous Paleocene Slumping Channel cuts Restoration of a typical toe thrust in the Krishna basin. Detachment along the top Cretaceous horizon. Basinward shift of growth faults and toe thrusts. Growth fault activity began after deposition of Paleocene, and continued to recent times. Slumping and channel cuts indicate continued shelf instability.

Stage 1 Undeformed Paleocene. Earlier deformations present

Stage 2 Beginning of linked system after Paleocene

Stage 3 Continued faulting

Stage 4 Continued growth activity

Stage 5 Restored seabed

Basement Cretaceous Paleocene Slumping Thrust throw unknown, eroded at top Channel cuts Stage 6 Present day topography with slumps and channel cuts

Restoration of typical shale bulge systems in Godavari basin. Detachment is along top Cretaceous horizon. Growth activity moves basinward, bulges moves landward. Distal bulge grows during Miocene and early Pliocene, but inactive through Pliocene to present. Proximal bulge grows through Pliocene. Restoration of a shale bulge in the Godavari basin

Unfaulted Early Miocene seabed; initiation of listric faulting at deltaic part

Late Miocene faulting along landward listric faults; initiation of distal bulge

Early Pliocene basinward shift of faults and initiation of proximal bulge

Present day topography LP = Late Pliocene, EP = Early Pliocene, LM = Late Miocene, EM = Early Miocene, P = Paleocene, K = Cretaceous, A = Albian, B = basement

Differences Krishna basinGodavari basin Massive growth sequences and toe thrustsShale bulges, shelf instability Growth in Pal-Oligo and L. Oligo-E. MioTwo phase growth: E.-M. Mio and E. Plio Pal-Oligo growth in southern part give way to L. Oligo-E. Mio activity E.-M. Mio activity give way to E Plio, producing two bulges; further distal bulge continues Steep basinward detachmentGentle basinward detachment Basinward progression of thrustsLandward progression of bulge Little control of underlying structuresControl of basement geometry

Detachment and activity timing

Major factors affecting DWFTBs in the KG basin  Loading rate and thickness of sediments above the detachment  Pore pressure distribution along the detachment  Slope of the detachment  Basinal tilt/uplift  Basement control; detachment slope break Factors Triggering cause  Oversteepening of shelf edge due to sedimentation

Sequence of events 1.First gravity-linked faulting → Oligocene in Krishna basin 2. Variable Upper Cretaceous-Upper Paleocene-Lower Oligocene detachment 3.Huge growth sequences with impressive toe thrusts 4.Second event → Late Miocene to Pliocene in Godavari basin 5.Upper Cretaceous detachment 6.Development of shale bulges 7.Associated with shelf instability in the basin 8.Intermediated areas show mixed patterns

Tectonic event chart

Thank you The ideas presented were developed in the course of the author’s involvement in the exploration activities of Reliance Industries Ltd. in the east coast of India. The author thanks Reliance Industries Ltd. for allowing to present the material.