Stratigraphic, Structural, and Correlation Considerations 37 



formity, but as initial dips may be high, a pseudononconformable rela- 

 tionship may exist. 



An aggrading stream building a flood plain of construction will oc- 

 cupy at one time or another every position in the deposit. This is ac- 

 complished through migration of meanders or by the stream breaking 

 natural levees and seeking the lower land of the back swamps. As a 

 flood plain often contains swamps and lakes, the unconformity developed 

 may be between fluvial, paludal, or lacustrine deposits. Unconformities 

 developed by aggrading streams have disconformable contacts and have 

 a short to moderately long time value. 



Alluvial fans are deposited by stream distributaries on surfaces of 

 varied origin, over which they extend outward from the uplands. The 

 buried surface becomes an unconformity that may represent a long or 

 short time value and may be either a disconformity or a nonconformity. 



Unconformities in deltas may be either continental or marine. A 

 delta may advance or retreat according to the balance between supply 

 and deposition. This advance and retreat leaves unconformable surfaces 

 which may separate fluvial, paludal, lacustrine, or marine sediments, 



Eolian : The work of the wind may result in unconformable contacts 

 of either type. Sand may be deposited by the wind on surfaces not pre- 

 viously affected to produce unconformities on burial. Winds may erode 

 earlier eolian deposits and later bury the eroded surface. Discordance 

 between cross laminations may give such a contact the appearance of a 

 nonconformity. The relief on eolian unconformities may be none to very 

 great and the time value small to large. 



Glacial: The advance of a glacier will be accompanied by erosion, 

 which tends to decrease the relief of the land surface over which it is 

 passing. Deposition of morainal material on this surface may result in 

 an unconformity of either type. 



Marine Unconformities. Marine unconformities are those found in 

 marine deposits or between marine deposits above and deposits of another 

 environment below. 



Overlap: Deposition of the basal sediments of a formation on an 

 erosion surface are not likely to be laid down uniformly and contem- 

 poraneously over a wide area. Actually, deposition will begin in a few 

 favorable places and spread to other areas as formations become thicker. 

 Thus succeeding strata will overlap the older rocks below the erosion 

 surface. 



As the land is submerged by the rapid rise of an encroaching sea, a 

 condition of overlap is created which has been called the "unconformity 

 of progressive overlap" by Grabau.^^ The coarse debris of the land sur- 

 face forms a basal conglomerate, and there may be buried soils. As de- 

 posits are made in the advancing sea, each depth is characterized by 



'° Grabau, A. W., Principles of Stratigraphy, p. 723, 1913. 



