Geological Contact: Sirohi Fold Belt And Erinpura Granite

One of  my favorite courses during graduate work in Pune was geological Remote Sensing. We focused a lot on visual interpretation of aerial photographs and satellite imagery. Lab work meant hours of poring over different areas of India and making preliminary geology maps based on observations of the landforms and rock structures visible from space. The rock composition, structure, and weathering patterns controls the surface expression of geological units. These are manifest on satellite imagery and on aerial photographs as differences in tone, texture, structural styles, and relief. 

I came across a nice example of the juxtaposition of two distinct geologic rock units just northeast of the town of Sirohi in Rajasthan. 

These two terrains made up of the Erinpura Granite and the Sirohi Group sediments are easily recognizable. 

The terrain west of the yellow line and isolated ribbons between the purple lines is made up of layered metamorphosed sedimentary rocks of the Sirohi Group. These sediments were deposited on the Erinpura Granite basement around 850 million years ago. Hard rock layers form northeast southwest oriented ridges with intervening valleys underlain by a softer lithology.  The texture appears fine and the ridge and valley terrain has a substantial vegetation cover. The rock layers can be traced as a series of folds. The Sirohi Group sediments were deformed and metamorphosed by 820 million years ago.

The Erinpura Granite represents magmatism that occurred around 900 million years ago.  Occurring mostly east of the yellow line, the granite also forms enclaves (orange outline) in the fold belt.

The granite appears as a light toned surface. It is massive body of rock, lacking the layering seen in the western sedimentary basin. The landform is a low elevation plain as compared to the Sirohi rocks.The rock has a coarse granular texture. Its surface has scant vegetation. Several lineaments (dark colored lines) are seen crisscrossing the granite body. These are fractures and dikes. Shrubs and trees growing along the fractures and dikes give them a darker appearance and make them stand out against the lighter granite body. The fracture sets have also controlled the local drainage. Several streams can be seen flowing along straight courses.

Below are two close up images of the contact zones between the two terrains. 

At this scale you can appreciate why the granite as a coarser texture in the synoptic imagery. The fracture systems has broken the granite in to a blocky surface. 

In this image below the Erinpura Granite occurs as an enclave surrounded by Sirohi metasedimentary rocks. 

And finally, a close up of structure controlled drainage in the Eripura Granite terrain. The fir tree like pattern, known as trellis drainage,  is due to streams flowing along orthogonal fracture sets. 

Geology mapping done. Now to visit Rajasthan for a field check!