570 



A. W. GRABATJ TYPES OF SEDIMENTARY OVERLAP 



2. Eegressive overlap. 



3. Compound regressive and transgressive overlap. 



Transgressive Overlap 



When the sea regularly advances upon an old land surface from which 

 there is a continued supply of detrital material, a steadily advancing 

 shore zone of pebbles or sand will be recorded in the sedimentary series 

 which is forming in the transgressing sea. At any given stage in the 

 process a shore deposit of coarse elastics, derived from the old land sur- 

 face, will form for some distance out, grading seaward into a deposit of 

 finer shore-derived material. In proportion to the distance from the 

 shore the fineness of the material will increase, and at the same time 

 material derived from organic deposits, such as coral reef sands or shell 

 formations, will accumulate in the regions of purer water. With pro- 

 gressive slow advance of the sea, the supply of detritus being uniform, 



A £ B 



Figure 1. — Diagram Illustrating Progressive (Transgressive) Overlap. 



the coarser shore elastics will be spread farther up on the old land, while 

 at the same time the zone of offshore deposits will migrate in the same 

 direction and approximately at the same rate as the shore itself. As a 

 result, the offshore deposits of a later period will come to rest on the 

 shore deposits of an earlier period, and if the transgression has been a 

 uniform one, on a uniform old land surface, with a uniform supply of 

 detritus, a vertical section of such a series of successive deposits will 

 show an upward gradation from coarse to fine comparable to the similar 

 gradation in texture of the deposit of a single period from the shore 

 seaward. At the same time there will be a continuous basal bed of 

 coarse elastics spread immediately above the old land surface within the 

 zone of transgression, this continuous bed being made up of the shore 

 ends of the successive units of the series formed during the successive 

 periods of the transgression. The basal bed will be essentially a lithic 

 unit, resting everywhere unconformably upon the old land surface, and 

 it will be succeeded upward by strata of similarly uniform lithic charac- 



