400 J. BARRELL RECOGNITION OF ANCIENT DELTA DEPOSITS 



Observation of river valleys and shorelines shows that crust movements 

 are relatively rapid between longer epochs of quiet. Stages in subsidence 

 will therefore result in wide and geologically rapid transgressions by the 

 sea over the subaerial delta plain. But such individual downward move- 

 ments are commonly small in amount, so that the volume of sea water on 

 the delta surface is correspondingly small and is soon excluded by the 

 readvance of the subaerial plain of the delta. The effect of individual 

 movements of subsidence is therefore to greatly extend for a short time 

 the subaqueous portion of the topset plain and result in an extremely 

 unstable shoreline. From this argument it follows that evidences of 

 either marine or continental origin of a certain stratum carry but little 

 implication in regard to the origin of inferior or superior portions of 

 the formation. This is illustrated by the Illinois coal measures, where 

 limestones holding abundant marine fossils are found repeatedly and 

 closely to overlie coal beds. Furthermore, a definite criterion of origin, 

 such as roots or mud cracks, if scattered vertically through a whole for- 

 mation, is seen to be much more significant of the dominant mode of 

 origin than is a unique stratum which nevertheless may be rich in evi- 

 dence. 



In order to bring the idea of the delta cycle to a workable agreement 

 with nature there must be considered the presence also of uplifts in the 

 crustal movements. Field investigations of the past decade have been 

 giving a continually larger place to disconformities, stages of lost record. 

 The Paleozoic seas were shifting water bodies and many times receded 

 from the land. Detailed work on the Mesozoic of the coastal plain shows 

 breaks representing long time intervals which separate the sevei-nl late 

 Jurassic and Lo^\-er Cretaceous fresh-water formations. The Pleistocene 

 crustal oscillations also involved repeated reversals of the dominant trend 

 in the movement. 



Applying the idea of upward phasesr in downward crustal movemculs 

 to the problem of delta gro^^i:h, it is seen that in the case of a delta with 

 distinctive foreset and topset beds a slight upward movement should re- 

 sult in a disconformity over the inland portion of the delta. The erosion 

 plane may vanish to seaward, giving place to land beds overlying parts 

 of the former subaqueous plain. A slightly greater movement, however, 

 will carry the disconformity to the foreset edge. In either case a great 

 increase in the volume of the marine foreset beds for a certain stage 

 marks the uplift of the delta and corresponds with the erosion and lost 

 record on its topset plain. If, as seems to have been a rather common 

 condition, the epicontinental sea was so shallow as to prevent the forma- 

 tion of a distinct foreset slope, then the slight regional uplift would 



