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What are the features of terrestrial lithographic/clastic sediment that indicate it was deposited into an oceanic as opposed to a terrestrial (e.g. fluvial or lacustrine) environment? How to tell whether a given siltstone bed was deposited in either environment?

Fred
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charlie_sar
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  • Homework questions are expected to show some attempt to answer the question on your own. – John Aug 07 '21 at 20:46
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    I'm studying this on my own, thanks, no one assigned homework. – charlie_sar Aug 07 '21 at 21:04
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    an example from Tulane U. EENS dept: "Fine grained clastics are deposited in non-agitated water, calm water, where there is little energy to continue to transport the small grains. Thus mudrocks form in deep water ocean basins and lakes." Nothing about differentiating oceanic vs. lacustrine deposition – charlie_sar Aug 07 '21 at 21:21
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    and this from U. of Houston / Geophysics does not clarify: https://uh.edu/~geos6g/1330/sedrx.html – charlie_sar Aug 07 '21 at 21:27
  • This might be useful Ocean Sediments – Fred Aug 08 '21 at 19:05

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Although now rather dated, USGS Bulletin 1080 has a useful discussion of various lines of evidence that can be used to distinguish lacustrine from marine deposits. Looking at purely physical characteristics it is often assumed that spatially extensive, fine grained, laminated deposits are suggestive of the low energy, and potentially seasonally varying, environment of a lake, with oceanic environments having greater energy, so less uniformity. It is easy, however, to see that there are may exceptions to this simple classification.

Fossil evidence can be a conclusive identifying factor, but isn't always available. The presence of evaporite minerals is suggestive of closed lacustrine environments, but obviously coastal sabkha evaporites are common.

Carbon/Sulphur ratio have been suggested as an indicator, with pyrite more common in marine sediments.

Andy M
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