Page 117 - Petrology of Sedimentary Rocks

Under quiescent tectonic conditions the top layer of the crust (sedimentary layer)
furnishes the detritus. Furthermore, under quiescent conditions, intensely matured
beach and dune sediments dominate the stratigraphic section. For both these reasons,
then, the resultant sediments will consist largely of quartz (with or without chert), i.e.,
they will be quartzarenites. If quiescence is prolonged, erosion may eat down to the
granitic layer, and some feldspar may enter the sediment.

Under orthogeosynclinal deformation, tectonic activity buckles the metamorphic
layer up to provide a source area. Intense folding, thrusting, and dominant horizontal
tectonic movements actually create low-rank metamorphic rocks like slates and
phyllites out of the folded sediments. Because of high relief and rapid erosion, the
dominant environments are those where rapid sediment dumping takes place, i.e.,
fluvial planes, deltas and neritic shelves. Thus sands are rich in metamorphic rock
fragments and are not well sorted; hence, they will be graywackes (now, 1966, termed
“Phyllarenites”). Fold mountains (Alps, Pyrenees, Slovak Carpathians) contain abundant
phyllarenites.

If violent tectonic movements of a dominantly vertical nature occur, as in block
faulting following a period of folding, the deepest layer of the crust, granite and gneiss,
is raised to the surface to provide a source area. Environments again are those of rapid
deposition, i.e. fluvial plains and alluvial fans. Sediments consequently are rich in
feldspar, therefore are arkoses.

Krynine’s theory is vastly oversimplified and idealistic, nevertheless it works
rather well in many areas and is an excellent guide to stratigraphic thinking. Its big
imperfection--and no model is ever perfect --is that it ignores the important part played
by the past history of a region, which is a large “chance” variable thrown into the ideal
tectonic scheme. For example, consider an area that was in some past age a
geosyncline filled largely with shale and that later became orogenized and converted to
low-rank metamorphics. If such an area is block-faulted, the pile of metamorphics
would be so immense that MRF-sandstones (graywackes of PDK) would be deposited
instead of, as theory proposes, arkoses.

Related to this is the absence of any place in the scheme for sedimentary deposits
resulting from simple rejuvenation (without differential deformation) of an old source
(of whatever Iithology). Another weak point is the very minor role assigned to volcanic
sediments.

In view of these relatively minor shortcomings to what is basically a sound
organized scheme, the following “New Gospel” (conceived in fall, 1965) is propounded as
a modification of Krynine’s scheme and to take into account these difficulties.

A Concinnity of Depositional Regions

To understand the relation between tectonics and sandstone types, one must first
consider gross continental architecture. The following scheme is a development of the
work of Stille, Knopf, Kay, Krynine, Krumbein-Sloss, and others. For the record, terms
originated by myself are shown thus (*).

In its early stages, a continent may consist of a stable kraton (typically mostly
granite or gneiss or high-grade metamorphics), surrounded in part by subsiding regions
of sedimentation (geosynclines) with outlying welts serving as most active sources of
detritus; most detritus moves centripetally, from the outlying welts towards the kraton.

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