Page 103 - Petrology of Sedimentary Rocks

(4) Hydraulic factor. Minerals of a certain shape or specific gravity will be
carried father away leading to changes in mineral ratios from specimen to specimen.
Thus, Brogdon and Bullard have shown that south of the Rio Grande, magnetite
decreases southward and hornblende increases southward due to different specific
gravity and shape, yet both are coming out of the mouth of the same river; if millions
of years later someone drilled a well through these sands, he might think they were
coming from a different source area when really it is only the hydraulic factor that is
operating. To evaluate this factor, plot mineral specific gravities against mineral
abundance in different areas; if hydraulic factors are operating, the points should fall in
a fairly straight line or smooth curve, with the platy or elongated minerals systemati-
cally displaced to one side with regard to the main trend.

(5) Post-depositional survival factor; on intrastratal solution by migrating
connate water, or on surficial weathering some of the less stable minerals (Garnet,
Pyroxene, Amphibole, Staurolite) may be destroyed or etched; to check this, compare
sealed (cemented) zones with porous zones. In Gulf Coast sediments, unstable minerals
appear only in the younger beds, possibly due to intrastratal solution of these minerals
in the older beds. Always look for sealed environments to eliminate this factor.
Pettijohn thinks this is a very important factor in heavy mineral occurence.

(6) Finally, a statistical error may be mentioned. Erroneous conclusions will
nearly always be reached unless standard statistical techniques are used.

All these extraneous variables (2 through 6) can be eliminated if one sticks to
varietal counts within the same size grains of the same species; these should have very
nearly the same durability, chemical stability and hydraulic behavior and so the only
variable operating will be the source area factor.

In studying a heavy mineral suite, one should (I) determine the character of the
entire suite, studying shapes, sizes, etc., to determine the general lithology of the
source area and study the effect of weathering and abrasion; (2) for detailed work, such
as distinguishing different areas of provenance within the same formation or else
distinguishing between or correlating two formations, use varietal counts on one or two
species, using 5 to 20 varieties per species.

Authigenesis. Magnetite, Tourmal ine, Zircon, Rutile, and Sphene can form
authigenically in sediments. Tourmaline and Zircon grow as overgrowths on detrital
grains, while the others usually form new crystals. Barite, celestite, siderite and pyrite
in heavy mineral suites are almost always authigenic.

Carbonate Minerals

There are a great many sedimentary carbonate minerals, but only three or four
are important rock-formers--Calcite, CaCa(C03)2; Dolomite, CaMg(C03)2; Siderite,
FeFe(C03)2; and Ankeri te, Ca(MgFe) (C03)2. The Mg and Fe ions are about the same

size and substitute for each other readily giving a complete range between (I) dolomite,
)
ankerite, and ferrodolomite, CaFe(C0 3 2; and (2) magnesite, several intermediate Fe-
Mg carbonates, and siderite. The Ca ion is larger, however, so that little substitution
normally takes place between Ca and Mg--slowly grown inorganic calcite contains at
most a percent or two Mg, and slowly grown dolomite contains at most a percent or two
excess Ca or Mg over the ideal formula. Organisms, especially those of more primitive
types such as crinoids, can build as much as I5 percent MgC03 into the calcite of their

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