42G J. BARRELL RECOGNITION OP ANCIENT DELTA DEPOSITS 



wave action. The preservation of perfect and fine-grained lamination in 

 many ancient fossiliferous marine muds seems to show tliat the muddy 

 ooze of deeper waters is not stirred by the bottom life to the same extent 

 as is true within similar sediments on the land. The smothering nature 

 of the deep, soft ooze is in fact unfavorable to most kinds of invertebrate 

 bottom life and may be correlated with tlie sparingly fossiliferous char- 

 acter of thinly laminated non-calcareous marine shales. 



Effects of waves. — Where muddy sediment is supplied to shallow waters, 

 as off the mouth of the Mississippi, the coast charts show intermixtures 

 of sand and mud, some parts of the bottom soft and others hard. The 

 waves of storms stir up such bottoms and shift its materials. The water 

 becomes discolored with sediment and considerable thicknesses must settle 

 on the subsidence of wave action. The stratigraphic result to be antici- 

 pated is a destruction of the fine and regular lamination of clays and 

 their intermixture with sand and silt. Such a massive structure in clays 

 is observed in certain fossiliferous marine formations — such, for example, 

 as the Merchantville clay of the Upper Cretaceous of the Atlantic Coastal 

 Plain, where bedding is characteristically absent except in the presence, 

 of laminae of sand. Where two materials of unlike nature, such as clay 

 and sand, are both present the results are quite different than in mud 

 deposits alone. 



Effects of subaerial actions. — On floodplains extensive pelitic deposits 

 are laid down in backwaters but little affected by currents, in shallow 

 lakes, and on the frontal parts of the delta. Where such clays are ex- 

 posed to the air various agencies tend to destroy the original lamination. 

 The effects of earthworms and roots are well known, but over regions 

 where the clays become mud-cracked a still more effective agency is in 

 operation. The cracks break across the bedding and in thick clays may 

 extend to depths of some feet. The next flood waters sweep more sedi- 

 ment into these cracks, the edges of the polygons slack and crumble and 

 the cracks become obliterated. The following period of drying opens 

 them again, but on more or less independent patterns. Thus the clay is 

 subjected to a thorough vertical mixing through a period of time re- 

 quired for an accumulation equal to the depth of the mud cracks. Where 

 the character of the sediment remains uniform, the filling is of the same 

 material as the dried polygons and there results massive clay formations, 

 in which both lamination and the evidence of mud cracking are absent. 

 The latter are commonly revealed only when sand or sandy clay has been 

 swept over the mud-cracked stratum, filling the cracks and protecting the 

 stratum from further action. It is consequently the bottom of sandy 

 gtrata resting on bods of shale which most commonly show the pattern 



