BORN IN SAND, BURRIED IN MUD: A DYNAMIC DEPOSITIONAL STORY
Casts of ?Thalassinoides burrows in siliciclastic succession, displaying a distinct color difference due to variation in energy and oxygen levels at the time of deposition of sand and mud respectively.
While it is more common for fossil burrows to form in finer sediment (like mud or silt) and then be infilled by coarser material like sand, the reverse scenario is also possible and can occur in certain depositional environments. This kind of sequence can happen when sedimentation conditions change quickly, such as in nearshore or shallow marine environments, where energy levels can fluctuate.
Notice the coarser grain size in the hematite stained arenite compared to the finer grain size in the dark gray mudrock.
The displayed sample illustrates burrows initially formed in a coarser-grained, possibly iron-rich, oxygenated environment indicated by the hematite staining, with the burrow walls preserved in the coarser sediment (sandstone) and later infilled with finer, dark-colored mudstone. The striking contrast between the red-stained, coarser burrow walls and the darker mudstone infill supports the idea of a high-energy environment followed by quieter, low-energy conditions. Here is how the process unfolded:
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Initial burrowing: Organisms created burrows in a sandy substrate, possibly because it was oxygenated due to higher energy levels and offered an ideal habitat for feeding or dwelling. The idea of high-energy environment, dominated by sand-sized grains deposition, is further supported by the presence of hematite, which typically forms in well-oxygenated conditions, like a nearshore or shallow marine environment where waves or currents brought in well-oxygenated water.
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Sedimentary environment change: A shift to a lower-energy environment might then deposit mud over the sandy substrate, filling in the burrows. This might have occurred during a quieter phase of sedimentation, like a calm period in a storm-dominated setting, or due to a change in water depth or current velocity.
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Cast formation: Over time, the fine-grained sediment that covered and filled the burrows lithified into mudstone, forming fossilized casts. The intact coarser-grained, iron-stained burrow walls and the mudstone infill were preserved, creating sharp visual contrasts in texture and color between the burrow walls and the infill.
This sample exemplifies how changes in energy and oxygen levels in depositional environments can create distinct and informative fossil burrow features. The hematite staining provides a valuable geochemical indicator of the original oxygenated conditions, suggesting a well-aerated setting with periodic shifts in energy levels. It’s a compelling example that offers insights into a dynamic paleoenvironment where burrowing organisms thrived in conditions that transitioned from high to low energy.
