Monday, March 18, 2024

Geological Contacts: Angular Unconformity Kaladgi Basin

 Remotely India Series #12

Through the Proterozoic Eon, beginning around 2 billion years ago,  extensional forces acting on continental crust opened up several sedimentary basins across what is now peninsular India. Crustal blocks subsided along faults and these depressions filled in with sediments deposited in fluvial and shallow marine environments. These basins were long lived, some lasting for more than a billion years. 

Sedimentation was not continuous.  Pulses of sediment deposition were punctuated by long periods of non deposition. Tectonic movements deformed early deposited piles of sediment. They were uplifted and an extensive basin wide erosional surface formed.

There was then a renewed phase of basin development. Sediment of these successor basins were deposited on tilted and folded older strata. Commonly, these younger packages of sediments are relatively undeformed. They are preserved as mesas and plateaus made up of flat lying strata. This discordance in attitude between two sets of strata separated by a widespread erosion surface is known as an angular unconformity.

In this post I will highlight an angular unconformity from the Kaladgi Basin from north Karnataka, south India. I have used high resolution imagery from Indian Space Research Organization's Cartosat.  Imagery is available for browsing and download from ISRO's Bhuvan 2D web maps.

The first image shows the area around Ramdurg village. The multi-stage history of the basin is readily apparent. The light colored strata exposed along narrow ridges are folded, while the rust brown hills are made up of undeformed sediments. The light toned strata are quartzites of the Bagalkot Group. The brown sandstone which rest on the Bagalkot quartzites are the Badami Group. Standard annotations show the varying dip and strike of the folded Bagalkot sediments. The white cross in grey circle denotes horizontal Badami strata. 

Kaladgi Basin history has become clearer based on recent geochronologic work by Shilpa Patil Pillai, Kanchan Pande, and Vivek S.Kale. They infer that basin initiation occurred around 1.4 billion years ago. Sedimentation of the Bagalkot Group terminated by 1.2 billion years ago. Movement along major WNW-ESE and tranverse NNE-SSE to NE-SW trending faults deformed the Bagalkot sediments into a series of folds around 1.1 billion years ago. This was followed by uplift and erosion of these folded sediments. Deformation was accompanied by low grade metamorphism of these rocks.

The basin floor subsided again around 900 million years ago initiating deposition of the Badami Group of sediments. The famous cave temples of Badami have been cut out from the lower part of the Badami sedimentary sequence.

The next imagery is a good example on how to recognize the relative timing of deformation events. Arrows point to fracture sets in the Bagalkot quartzites. These lineaments do not extend into the Badami sediments implying that fracturing occurred during an earlier phase of deformation. 

Let's look at a location that shows the angular discordance between the Bagalkot and Badami sediments. This is near Shirur town, north of Badami.  The lighter toned steeply tilted Bagalkot sediments outcrop as E-W trending narrow ribbons, north of Budanagad village. The brown colored Badami sediments form a more extensive plateau. Since these strata are horizontal, the traces of bedding planes form concentric bands mimicking contour lines. 

The final location is just south of Ramdurg village. The unconformity here is a little harder to decipher, but you can make out the tilt of the light colored Bagalkot quartzites, annotated by the standard notation of strike and dip. The quartzites form triangular facets sloping eastwards. Like the previous example, the concentric bands of brown in the adjacent hill indicates that this is the overlying horizontally disposed Badami sandstone.

Many Proterozoic basins of India contain such unconformity bounded sequences. Some more classic examples come from the Chattisgarh, Cuddapah, and Vindhyan basins. These sequences from different basins were not deposited synchronously. Each basin has it own trajectory of sedimentation, deformation, and erosion. 

Detailed field mapping, supplemented by absolute dating of rocks wherever possible, is elucidating the complex poly-phase history of Indian Proterozoic sedimentary basins in the context of global continental breakup and reassembly. For arm chair geologists and enthusiasts, easily available web mapping technology makes it possible to join in the excitement of teasing out these terrain's many secrets hiding in plain sight.

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