Vj. Hurst et Sm. Pickering, ORIGIN AND CLASSIFICATION OF COASTAL-PLAIN KAOLINS, SOUTHEASTERN USA,AND THE ROLE OF GROUNDWATER AND MICROBIAL ACTION, Clays and clay minerals, 45(2), 1997, pp. 274-285
Along the inner Coastal Plain, kaolinite-metahalloysite-rich, neritic
muds of Cretaceous-Eocene age have undergone intense postdepositional
alteration in the recharge area of the regional groundwater system. We
athering processes have had the following profound effects on the orig
inal sediments: 1) strong compositional and textural modification of b
oth clay and non-clay minerals; 2) whitening of the originally darker
sediments by partial removal of organic matter, Fe and Mn; and 3) recr
ystallization of kaolinite and metahalloysite, most conspicuous where
there are coarse stacks and vermiforms. Where the combination of initi
al sediment composition and alteration intensity was most favorable, t
hese changes have produced important deposits of commercial quality, w
hich now sustain the world's largest kaolin production district. The e
arliest change was partial sequestration of iron as sulfide and concur
rent destruction of some organic matter, mediated by sulfate-reducing
bacteria. Subsequent weathering resulted in gradual leaching of alkali
es, alkaline earths, iron and silica, and attendant nucleation and gro
wth of minerals compatible with the compositional changes. The existen
ce of several closely spaced erosional unconformities, separated by ne
ritic sediments, is proof that weathering conditions commonly changed
at a given site, in response to changes in thickness or lithology of t
he overlying rocks. Dsyoxic --> <-- oxic reversals modified both the r
ate and kind of alteration. (''Dysoxic'' refers to molecular oxygen co
ncentration too low to be toxic to anaerobes or cause abiotic oxidatio
n; less extreme than ''anoxic''.) Kaolins were produced partly by slow
er dysoxic weathering in saturated groundwater zones but mainly by mor
e rapid oxic weathering in unsaturated zones, where bauxites also loca
lly formed. Gradual transformation of some sediments to kaolin rarely
began and ended in the same epoch. At several places most of the kaoli
nization (see ''Definitions'') took place during Recent time, tens of
millions of years after deposition of the sediments. Since the kaolins
resulted from postdepositional alteration rather than sedimentary pro
cesses, they are better referred to as ''Coastal Plain'' rather than '
'sedimentary'' kaolins.