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 :)
Very fascinating!!
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