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Ways of Deciphering Compacted Sediments
Analyzing the effects of compaction and differential compaction can give "wholesale returns of conjecture. One kind of analysis depends on calculating a "decompaction number" which, when multiplied by present thickness of a compacted interval, will yield the original thickness. The simplest parameter for this is grain proportion (complement of porosity, or dry bulk density divided by solid grain density), because thickness times grain proportion remains constant during compaction of a bed. The decompaction number for clayey sediments is the present grain proportion divided by earlier or initial grain proportion. Initial grain proportion averages 0.22 (78 percent porosity), and solid grain density averages 2.66 gm/cc.
Other kinds of analysis are aided by a graph of porosity versus depth of burial. The graph is a composite of published data on compaction tests, drill cores, and cores of unlithified clayey sediments.
Differential compaction lends itself to empirical analysis, especially where compacted coat or shale inter-tongues with noncompacted sand. Tentative decompaction numbers for coal are cited from unpublished data.
Several conjectures are drawn from compaction features in southwest Vermont, involving Cambro-Ordovician argillites and distal sandy interbeds of an inferred submarine fan. Shallow scouring is revealed by restoring initial geometry. The decompaction number argues against tectonic thinning. Porosity of the argillites was perhaps 30 percent during gravity sliding of this slice of the Taconic klippe, was reduced by tectonic overburden, and was eliminated by mild Acadian metamorphism.
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