DIATOMITE 



193 



Figure 23. — Estimated diatomite production, 1948-68, 

 adapted from production fibres and estimates by the 

 U.S. Bureau of Mines (1958-71). The pronounced upturn 

 in the world production curve between 1956 and 1960 

 mainly reflects increased production figures for Denmark 

 and France and larger production estimates for the 

 U.S.S.R. 



and the concentration of biogenous silica in their 

 accumulated remains are major factors in the silica 

 balance of most water bodies. Diatoms live in nearly 

 all aquatic environments, but they form thick pure 

 deposits only vi^here they ilourish for a considerable 

 time avi^ay from contaminating sediment sources. 

 Preservation of the siliceous parts of diatoms may 

 be favored in sediments high in organic matter 

 (Berger, 1970, p. 1387) ; also, the inclusion of di- 

 atoms in fecal pellets may enhance the chance of 

 preservation of diatom frustules because the mem- 

 brane that encases a pellet protects the frustules 

 from dissolution, and the pellets speed the fall of 

 frustules through the water column (Schrader, 1971, 

 p. 56-57). 



Because diatoms growing in abundance soon de- 

 plete the silica supply of near-surface water, sus- 

 tained diatom production depends on continued 

 renewal of silica-rich water to the surface. The 

 interaction of many factors determines the abun- 

 dance of diatoms at any particular place and time, 

 but commonly diatoms proliferate in marine areas 

 of upwelling, in lakes on volcanic terranes, and in~ 

 lakes in areas that had Quaternary glaciers. Al- 

 though marine diatomite commonly is associated 

 with volcanic rocks (Bramlette, 1946, p. 39; Khvo- 

 rova, 1968, p. Ill; Tahaferro, 1933, p. 36), at some 

 places oceanic circulation alone supplies sufficient 

 silica to account for thick accumulations of diatoma- 

 ceous sediments without requiring introduction of 



additional silica from nearby volcanism (Calvert, 

 1966, p. 593). The beneficial effects of an ash fall 

 on the diatom population of a modern lake have 

 been shown (Kurenkov, 1966, p. 429), but silica 

 seldom is a limiting factor in diatom production in 

 lakes except during intensive diatom blooms 

 (Schelske and Stoermer, 1971, p. 423). Papers by 

 Gregor (1968), Grill (1970), Harriss (1966), 

 Krauskopf (1956), and Siever (1957) consider the 

 role of biogenic silica in maintaining the silica con- 

 centration of natural waters. 



Marine diatoms occur in rocks as old as Mesozoic, 

 but with some notable exceptions (Hanna, 1927), 

 thick deposits of marine diatomite are middle Ter- 

 tiary or younger. Nonmarine diatoms occur in rocks 

 as old as late Eocene (Lohman and Andrews, 1968), 

 but most nonmarine diatomite deposits are late 

 Tertiary or Quaternary. The lack of more ancient 

 diatomaceous rocks may be due partly to conversion 

 of the opaline silica of diatom frustules to other 

 forms of silica (Ernst and Calvert, 1969, p. 131). 

 Marine Tertiary porcelaneous rocks that occur in 

 great thicknesses in California almost certainly 

 represent altered diatomaceous rocks (Bramlette, 

 1946, p. 50; Lohman, 1960, p. 184-185). 



Economic diatomite deposits are of three main 

 types : ( 1 ) marine rocks that accumulated near con- 

 tinental margins, (2) nonmarine rocks that formed 

 in lakes or marshes, and (3) sediments in modern 

 lakes, marshes, and bogs. Diatomite deposits near 

 Lompoc, Calif. (Industrial Minerals, 1969, p. 14-16, 

 17, 37; Mulryan, 1936), exemplify the first type; 

 this material formed in a shallow-marine environ- 

 ment in late Tertiary time. Similar marine diatomite 

 deposits occur elsewhere in coastal California, and 

 less pure marine deposits occur extensively in Mary- 

 land and Virginia. Diatomite deposits that formed 

 in lakes or marshes in Nevada, Oregon, Washington, 

 and eastern California represent the second type, 

 and diatomaceous sediments in modern lakes, 

 marshes, and bogs in Florida, New Hampshire, and 

 New York (Conger, 1939) belong to the third type. 

 Modern marine sediments are a potential fourth 

 source of diatoms. 



PROSPECTING TECHNIQUES 



Prospecting for diatomite involves first outlining 

 of areas that contain stratigraphic sequences likely 

 to include diatomaceous strata — for example, the 

 Tertiary and Quaternary lake basins of the Western 

 United States, or the Miocene marine basins of 

 coastal California — followed by examination of out- 

 crop sections in these areas. After diatomite is 

 found, the initial reconnaissance is followed by 



