CHAPTER XVIII. 

 OVERLAP RELATIONS OF SEDIMENTARY FORMATIONS. 



Overlap, or the extension of one formation beyond the other, 

 is a structural feature of the greatest significance in stratigraphy. 

 Two kinds -of overlap may be recognized, the irregular and the 

 regular or progressive. The irregular overlap of strata, resulting 

 from sudden changes of physical conditions, is more of the nature 

 of an accidental feature, and belongs rather to the general subject 

 of unconformity of formations. While its recognition is of great 

 importance in establishing the progress of events, it has not the 

 stratigraphic significance of the other type. 



PROGRESSIVE OVERLAP. 



^nder this term we include the regular overlapping of suc- 

 cessive formations due either to a normal transgressive movement 

 or tp a regular regressive movement. According as we deal with 

 marine or non-marine sediments we have (Grabau-3) : (A) The 

 marine progressive or (B) the non-marine progressive overlaps. 

 The former are the more varied in type and will be discussed first. 



A. MARINE PROGRESSIVE OVERLAP. We may dis- 

 tinguish two types of marine progressive overlap: i. Normal 

 transgressive overlap, due to a progressively encroaching sea ; and. 

 2. Regressive or retreatal overlap, due to a progressively retreating 

 sea. Generally a regressive movement is both preceded and fol- 

 lowed by a transgressive movement, so that as a result of this 

 compound progressive overlap a complex type of structure comes 

 into existence. 



The progress of depoaition of the clastic sediments on a normal 

 shelving seashore is controlled by two factors : namely, the rate of 

 supply of material, and the rate and direction of change in the rela- 

 tive position of sea-bottom and sea-level. According to the varia- 

 tion of one or both, sedimentation will vary. 



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