ORIGIN AND CLASSIFICATION OF COASTAL-PLAIN KAOLINS, SOUTHEASTERN USA,AND THE ROLE OF GROUNDWATER AND MICROBIAL ACTION

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.