Tuesday, June 20, 2023

Field Photos: Folds- Tectonic Or Soft Sediment Deformation?

During my wandering in the high valleys of Kumaon I have been noticing some intriguing boulders of sandstone which have rolled down from the high ridges in the vicinity of Panchachuli Glacier and Milam Glacier. I have access to only these blocks and for now have not been able to observe them in outcrop.

But they do show some spectacular deformation as seen in the picture below.

Is this folding due to tectonic forces that crumpled up the sedimentary sequence in to a series of small scale folds much later in the history of this region, perhaps during Himalaya orogeny?. Or, is this soft sediment deformation that took place contemporaneously with the deposition of these sedimentary layers? This occurs when semi hardened sediment moves and slides down slopes during earthquakes, contorting layers into small folds.

At this point I have not been able to decide which of the two mechanisms is at play here. What these blocks do demonstrate very nicely is how materials with different rigidity respond differently to an applied force.

Look at the sandstone boulder below.

The folding is restricted only to the central part of the block. This may happen because that layer is soft enough to flow under stress. The more rigid layers enveloping it don't warp. 

Here is another example. Notice the tiny white veins (black arrows). They occur only within a thin rust colored layer. These to me appear to be tear cracks. These small fractures open up under the influence of localized tensile forces. The rust layer was hard and brittle enough to crack while the surrounding material was too soft.

Although I haven't been able to see these sandstones in outcrop, I do get the feeling that this deformation is very localized and doesn't affect the entire sequence. This is because I have observed blocks with undisturbed thin layering, like in this cross bedded slab below. This hints that intermittent soft sediment deformation is the likely explanation, but I can't say for sure without examining the layers in outcrop.

 And this final picture of a large boulder with some crazy contorted layers. 

This is why geology field work is so much fun. You keep coming across these puzzles that make you think and which stay with you long after you have packed up camp and returned home. 

Looking once again at these pictures, I am desperate to head back. And there is more to understand about these boulders. I'll be writing about their stratigraphy i.e., their position within the thick rock unit known as the Tethyan Sedimentary Sequence, and their significance in understanding the geologic events occurring along the northern margin of the Indian subcontinent in the early Paleozoic, about 500-475 million years ago.


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Tuesday, June 13, 2023

Links: Human Brain Evolution, Pyrometamorphism, Upper Atmosphere Cooling

I learned some new things from these articles over the past couple of weeks.

1) Endocranial Volumes and Human Evolution: Warning- this figure posted below is deceptive!

Although hominin brain volumes increase over a 7 million year history, patterns of growth in separate lineages show, both, stasis & episodic increase. In an excellent analysis, anthropologist Ian Tattersall shows that a trend towards large brain volume is expressed independently in three separate hominin lineages, raising important questions about the role of social interactions and environmental pressure that could lead to the evolution of larger brain size. And most intriguingly, brain volume size has decreased in the Homo sapiens lineage over the past few tens of thousands of years. What does it mean for the evolution of complex behavior and symbolism?

2) Scorched Minerals in Sedimentary Rocks: Petrologist Michael Anenburg reports a most unusual suite of minerals. They formed by pyrometamorphism, i.e., the transformation of sedimentary rocks by heat supplied by large fires. This process takes place at or very near the surface, likely driven by the ignition of oil bearing shales or coal seams. The rocks described here are from the Dead Sea area of Israel. Before metamorphism, they were a sequence of impure limestones and phosphorites. There is a memorable description of these combusted limestones in the paper; 

" Gross discovered that the Hatrurim Formation was fundamentally a natural Portland cement factory. Indeed, many of the synthetic compounds found in cement occur naturally in the Hatrurim Formation and were subsequently named after the local Hebrew or Arabic place names in which they were found, such as hatrurite, ye’elimite, and harmunite. Concrete is formed when Portland cement is mixed with water, and the pyrometamorphic minerals of the Hatrurim Formation have experienced a similar process. Hundreds of thousands to millions of years of exposure to rain and groundwater has led to the hydration and alteration of most of the high temperature minerals. The end result is essentially a naturally formed concrete". 

3) The Upper Atmosphere Is Cooling, Prompting New Climate Concerns: The earth's atmosphere is layered. While the lowermost  layer known as the troposphere is warming as we emit more and more carbon dioxide, satellite data shows that the two uppermost layers, the mesosphere and the thermosphere have cooled by 3.1 deg F between 2002 and 2019. Scientists worry about the impact of this cooling on weather patterns on earth. A succinct summary by Fred Pearce.


Author Contact: suvrat_k@yahoo.com