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What could be the reason for forming this kind of structure‑ Could it form in a glacial context?

The structure is isolated within a massive diamictite. The core is well-sorted sandstone, and the rim is clast-supported conglomerate. It doesn't look like typical fluvial or debris flow features I know. Given the glacial context of the overall formation, could this be an ice-marginal feature, like a subglacial cavity fill or a lodged boulder with a meltwater moat? What diagnostic criteria should I look for?

 

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By Vladimir Answered 1 year ago

That's an excellent and diagnostic field observation. I have seen similar structures in Pleistocene tills and ancient glacial sequences. They are often interpreted as liquefaction features or "pillows" where a cohesive block of sand (perhaps a rafted sediment clast) sank into water-saturated, unconsolidated diamict. The conglomerate rim could represent coarser material that settled around the sinking block. To test a glacial origin, examine the clast fabric in the rim and the surrounding diamictite; a preferred orientation would support subglacial shear. Also, check for striated clasts. This structure strongly points to a wet, deformable subglacial bed.

 

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