Thread by @rapiduplift

Natural History in Temple Building Stones.

Thread- Sedimentary depositional environments of Neoproterozoic age (900-800 million yr old) building stones of Aihole, Pattadakal and Badami temples, rock cut caves and sculptures.

#Karnataka

Between ~1800 -800 million yrs ago, Indian continental crust sagged due to various tectonic forces to form several long lasting sedimentary basins. The Kaladgi Basin in which the Badami area sandstones were deposited is one such basin.

Map- ias.ac.in/article/fullte…

Much of this deposition took place in inland seas that flooded the Indian continent. During intervals of sea level fall, rivers carved valleys and deposited coarse sediment. The Badami Cave sandstones are river deposits of the Kaladgi Basin.

Geology - ias.ac.in/article/fullte…

Geologists have recognized that the Badami sandstones were deposited in a large braided river system with a north westerly flow. It was receiving sediment eroded from Archean age (>2.5 billion yr old) rocks situated S & SE of the basin.

Land plants did not exist then. Weathered debris was moved quickly by surface flow into streams. Large sediment load chocked channels, forcing bifurcation of streams and formation of braids. Broad braided rivers formed since there were no plants to stabilize banks.

e.g from NZ

Detailed sedimentological analysis of the Badami rocks has led to recognition of the various sub-environments of this paleo-river. Pic shows channel and sand bar deposits.

Source- Mukhopadhyay et. al 2018 in Geological Evolution of the Precambrian Indian Shield.

As river channels episodically migrated sideways and the valley floor subsided to accommodate more sediment, channel deposits and adjacent sand bars got stacked to form thick 'multi-story' sandstones. Each bed tells a story of a discrete depositional episode.

The arrangement of sand layers within each bed tells us about the subenvironments in which it formed and the energy and direction of water flow during deposition. These r called; tabular cross beds, trough cross beds, planar lamination, rippled beds.

Water (or wind) can move sand into waves. Sand grains roll along direction of flow, then avalanche down steeper side of the wave forming a layer inclined (cross) to the orientation of the main sand body. Successive avalanches form a set of cross beds.

serc.carleton.edu/NAGTWorkshops/…

Example of cross beds from near town of Badami (left) and from a building stone from Pattadakal temple (right). Such cross beds were built by sediment avalanching on the lee side of migrating sand bars during high flow.

Pic show trough cross bedding from near the Badami cave complex. These represent the internal structure of migrating sinuous sand dunes on a channel floor. See this elegant explanation by @sumnerd on how trough cross beds forms.

@sumnerd And here is a beautiful example of trough cross bedding found in a Pattadakal temple building stone.

@sumnerd Planar Lamination; Bed seen in pic constructed of parallel layers of coarse sand. Interpreted to have been deposited in a high flow regime from sheets of water flowing over mid channel bars.

Pattadakal.

@sumnerd Ripples on slab at Pattadakal. This is a rare preservation of a bedding surface showing rippled sand. Erosion usually cuts off the wavy upper part. Indicates migration of small sand waves in a quieter flow regime on the channel floor.

@sumnerd Small cross sets on floor of Aihole rock cut temple! These represent the cross beds formed by migration of small ripples. The ripples themselves have been eroded away. Arrows indicate the direction of water flow and cross bed accretion as ripples migrated.

@sumnerd This slab is geologically upside down! How to tell?
As sand avalanches down the lee slope it forms a tail at the toe of the slope resulting in cross beds which become tangential to the floor. In pic x beds are tangential towards the top of slab i.e. that is actually the base.

@sumnerd Original depositional orientation of cross beds manifest in this outcrop near Badami caves. Shows a tail or tangential contact of the cross beds with the base. Since top of cross beds r not usually preserved they show a high angle contact truncated by upper bedding plane.

@sumnerd This slab is upside down too! Notice again the tangential contact of the cross beds is towards the top (white arrow), which means that must have been the base. Yellow arrow points to high angle contact with the upper bedding surface.

@sumnerd Quiz- Is this slab geologically right side up or is it inverted?

@sumnerd The outcrops and building stones of these monuments record the processes within the Badami braided paleo-river. 900 million yrs ago a complex of channels and bars, quieter pools and rippled sand beds existed where these temples stand today.

@sumnerd Do visit Aihole, Pattadakal & Badami & gaze at its splendid architecture and sculptures. But spare some time to appreciate the magnificent record of our natural history that these monuments preserve.

Quiz- Is this slab inverted or depositionally right side up? 😉

End Thread**