SECT. 3] BASIN SEDIMENTATION AND DIAGENESIS 587 



c. Rafted 



These are of two types, those rafted by organic matter and those rafted by 

 ice. Such deposits can generally be recognized by the occurrence of erratically 

 distributed large pebbles or even boulders. Their origin can sometimes be deter- 

 mined — rounding and smoothness in gastrolytes, striations in ice stones. The 

 importance of ice-rafted pebbles may be great in higher latitudes, especially 

 where glaciers reach the sea. 



d. Residual 



Debris obtained from in situ weathering of rocks cropping out on the sea 

 floor. These may be igneous or metamorphic basement rocks or ancient sedi- 

 ments exposed by erosion. Grippenberg (1939) describes outcrops of Silurian and 

 Ordovician limestone in the Gulf of Bothnia, while Arkhangel'sky and Strakhov 

 (1938) describe areas in the deej) portions of the Black Sea where considerable 

 areas of dark grey-black calcareous mud with organic contents of over 20% 

 exist. These deposits belong to the Euxinian time, probably the Wiirni glacial 

 period. In this case detrital deposition has probably been slow so that it has not 

 covered the older deposits, while organic matter settling from the surface may 

 have enriched them and a complex history of subaerial, subaqueous and sub- 

 marine erosion may be involved. Tojiographic highs on the bottom of many 

 basins may be due to outcrops of ancient beds covered by recent sediments. 



e. Relict 



These are ancient deposits formed under different environmental conditions 

 than those now present, but which have remained uncovered by recent sedi- 

 ments. The most important are ancient sand dunes and beach ridges formed 

 during the Pleistocene and now covered by as much as 100 m of water as a 

 result of rise in sea-level. Uncovered glacial tills could conceivably be included 

 in this category since ancient glaciers, extending into relatively isolated basins, 

 may have deposited shallow-water sediments that are now covered by greater 

 depths of water. 



f. Authigenic 



Sediments that are chemically or biologically deposited in situ make up this 

 category. In stagnant basins or those with reducing conditions in the sedi- 

 ments, metal sulfides (e.g. pyrite) are important. In mildly reducing sediments, 

 metal carbonates (e.g. siderite) can form, while under oxidizing conditions metal 

 oxides will form. Carbonates, silicates and probably divalent cation sulfates 

 will form in arid climate basins, principally lagoons, by evaporation. Complex 

 silicates such as tourmaline, chlorite and glauconite along with phosphorite 

 are known to exist in basins, often associated with organic matter, 



g. Organic 



Sediments of this class can be broken down into two categories. The first, 

 of an organized Jiature and largely inorganic in composition, is derived from 



