Iron Pisolites From Western Ghats

A reader sent me this photograph of a pebble he had collected from a stream bed near Belgaum, Karnataka.

 Photo credit: Gopisundar

These look to me like iron rich pisolites. These spheroidal grains form by the accretion of iron, manganese, and aluminum hydroxides around a nucleus. The core may be an aggregate of soil particles, a rock fragment, or even wood debris. 

You will notice that the core is quite massive and structureless but in a few grains a crude concentric layering is seen at the margins. The pisolites are bound together into a firm mass by mixture of clay and iron aluminum hydroxide. Pisolites form during prolonged episodes of chemical weathering of  rocks like basalt or shale or iron aluminum rich metamorphic rocks. They are present in thick laterite and bauxite profiles. 

The picture below is a representative example of pisolite from a location in Brazil. It shows the occurrence of pisolite layers in a weathered soil profile, along with a hand sample and a cross section under high magnification.

 

Source: K Marques et.al. 2022: Geochronology (preprint).

Here is a map of the landscape just south of Belgaum that I am describing in this post. 

 

 Source: Amanda Jean et.al 2020: Journal of Geological Society

It shows the distribution of three distinct horizons of chemically weathered soil, named as the S1, S2 and S3 surfaces. Each of these horizons consist of tens of meters of laterite or bauxite and manganese rich ore. There has been some recent success in dating these weathered layers using the mineral crytomelane, a potassium rich manganese oxide. Three distinct weathering periods are documented. As India broke away from Gondwanaland it eventually drifted northwards into tropical climatic belts. Throughout the Eocene to Miocene, long phases of hot humid climate resulted in intense chemical alteration of the Western Ghat landscape. 

The oldest soil, surface S1 in the map, formed between 53-44 million years ago. Surface S2 formed later in the Oligocene-Miocene between 39-22 million years ago. And surface S3 developed in the mid Miocene, between 14-10 million years ago. 

The graphic below tells a story of landscape evolution recorded in the formation of these three weathering profiles. 

 

 Source: Amanda Jean et.al 2020: Journal of Geological Society

Episodic dissection of a plateau through the Cenozoic kept stripping away rock layers, and younger bauxite and manganese rich soils formed at lower altitudes on freshly exposed rock and debris flows. The youngest surface S3 has developed on a pediment. These are layers of debris eroded from surrounding hills that accumulate in low lying areas. Surface S3 indicates a very active phase of weathering and erosion of the surrounding mountains ranges that took place between 22 and 14 million years ago. As a once contiguous plateau was fragmented, older surfaces S1 and S2 remain preserved on isolated mesas and table lands.

The pisolite my friend sent me could have broken off from one of these surfaces. Its hard to say which one. The overall light color suggests that it is aluminum rich, and may have been sourced from the bauxitic S1 or S2. But this is just a guess. 

Pebbles from a stream can hold many secrets. Don't just chuck them away :)

Map Of Red Sea Corals

From my Nature News Feed- A great map of Red Sea coral reefs.

Credit: Gwilym Rowlands

This is a fine example of habitat mapping done on a resolution finer than previous examples and focusing not just on living corals but taking a historical perspective and exploring how sea-level change, climate and weathering influenced the substrate conditions forcing new coral growth to adapt and shift their locus.

Tafoni Occurs In Basalts Too

First things first. Go over to Michael Welland's blog - Through the Sandglass and congratulate him. His superb book Sand:The Never-Ending Story has been awarded the John Burroughs medal for excellence in natural history writing.

Sometime back Michael wrote a couple of posts on Tafoni, the beautiful enigmatic natural rock weathering sculptures found commonly in sandstones but also in other types of lithologies. The images that stuck with me from his posts were of desert landscapes and sandstone sculptures....so I got a pleasant surprise when I came across Tafoni in basalt on my recent field trip to the west coast.

Tafoni are ellipsoidal..bowl shaped natural rock cavities...go here to learn more.

They commonly occur in locations where salt weathering is intense. Water percolates through natural cracks in rocks and precipitate salts. These crystallizing salts exert pressure on the surrounding rock and little by little the host rock gives way to form holes and pits. Besides salt weathering the natural variation in hardness in rocks masses may also promote this differential weathering.

The location was just south of the town of Alibag ( Alibag incorrectly shown...it is directly west of the annotation... on the coast), near Korlai fishing village.

There are wave cut basalt benches exposed at low tide which are intruded by dikes. These basalt flows are studded with secondary silica and zeolites mineralization and ...Tafoni.

Here are some examples:

A close up of the beautiful honeycomb structure

Pits and closely spaced joints..

Associated silica and zeolite geodes

Pits...large and small..

Cluster of Tafoni on vertical and inclined surfaces..separated by regions which lack Tafoni..

Besides being located in the intertidal zone where constant wetting and drying and salt weathering is strong..there may be a couple of other reasons why Tafoni is localized to these flows.

First, these flows are not one homogenous rock mass. They are compound flows...i.e. made up of smaller flow units...small and big blobs of lava that overlap, interfinger and coalesce to form a larger compound flow representing one macro eruptive episode. That means there are flow unit scale variations in cooling rates and vesicular content that makes individual flow units unequally suseptible to weathering.

And second,  is the presence of dikes. Not directly..but these dike swarms are concentrated in regions where the crust has undergone extension...leaving the rocks riddled with closely spaces fractures.

That makes them vulnerable to dissolution through circulating water and chemical weathering.