Page 136 - Petrology of Sedimentary Rocks

the field. McBride wishes to define it compositionally as having over 15% chlorite/
sericite matrix, over 10% unstable rock fragments, and over 5% feldspar. It is a
special, polyvariant term in which hand-specimen properties are more important than
the precise composition or, texture.

Preferred Combinations of Sedimentary Types

Following are discussed the preferred associations of tectonic, mineralogic, and
environmental factors which form the different types of sandstones. Again, this list is
an excellent guide but must not be taken as gospel as there are some exceptions.

Petrology of Arkoses and Related Rocks

Feldspar, the essential mineral of the arkose, is comparatively easily decomposed
by weat hering. Therefore, in order for it to be abundantly presented in a sediment,
special conditions are required. Either (I) climatic conditions were too dry (or too cold)
to allow much weathering, and the feldspar remained largely undecomposed, or (2) the
source area was uplifted and eroded so rapidly that sufficient time was not available for
weathering to be completed. Type (I) is here designated the climatic arkose, because it
owes its high feldspar content to a dry or cool climate; type (2) is the tectonic arkose,
because it owesits feldspar content to a rapid tectonic uplift. A third type, the
volcanic or plagioclase-rich arkose, owes its feldspathic constituents to volcanic
activity, again a case of rapid deposition short-circuiting weathering.

Tectonic Arkose (Krynine), often Dv.G/p/CF(w). If violent vertical deformation
and block-faulting of the order of several thousand to 20,000 feet take place, granitic
or gneissic basement is characteristically raised to the surface. This type of tectonism
is typically associated with KA tectonics (and aulacogens, which are incipient KAK).
Continued sporadic movement maintains a rugged topography, into which vigorous
streams carve deep valleys. Rapid erosion on steep slopes contributes a huge load of
sediment, which quickly piles up as coarse alluvial fans passing out into aggrading
fluvial plains; sediments are very thick (thousands of feet) near the source, but thin
rather rapidly outward so that the form of the seidmentary body is that of a wedge.
Most of the described examples have formed under a warm, seasonally humid climate,
but there is no reason why such an uplift could not happen in areas of dry climate also.

Near the source, the alluvial fan environment dominates so that sediments are
quiet coarse (conglomerates common) and poorly sorted (clayey, immature) although
local stream channels may have better sorted sands. Farther from the source, deposits
consist of alternating beds of river-channel sand (which may be mature arkose, i.e., well
sorted), and immature flood plain muds and muddy sands with little or no gravel.
Composition may range up to 50 percent or more feldspar (mainly K-feldspar); Krynine
has shown that because of rugged relief combined with humid climate, some feldspar is
eroded from fresh granitic bedrock in the bottoms or sides of vigorous down-cutting
stream channels, and other feldspar is eroded from interfluves where it has had a
chance to be thoroughly weathered in the soil mantle. Thus the diagnostic marker of
the tectonic arkose is a mixture of fresh plus badly weathered feldspars of the same
species. The remainder of the rock consists largely of quartz; clay (often kaolinitic or
bauxitic) is abundant either as a matrix to the immature sands or as abundant
intercalated shale beds; it is provided by the thick intensely weathered soils. Heavy
minerals are abundant and of unstable types, such as hornblende, garnet, pyroxene, etc.
Cements are not common because sands are often clogged with clay matrix and these

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