Patterns of deposition at the continental margin 299 



namely the deep basins and shallow inter-island platforms of the East Indies. This, 

 however, is a completely different structural picture, and we will return to this case 

 later. 



The investigation of these submerged borderlands, their sediments, topography 

 and structure, is one of the main fields of endeavour in submarine geology today. 

 It should be clearly borne in mind that the present continental slope, however formed, 

 is only a topographic feature and does not represent any sort of boundary between 

 hypothetical continental and oceanic blocks of the crust. In fact, recent seismic 

 work off Nova Scotia and Georges Bank (Officer and Ev/ing, 1954) would seem to 

 suggest that there is no difference in the density of the basement complex which 

 underlies this continental terrace, at least to the limits of this particular survey (Fig. 1). 

 The terrace itself is made up of three major divisions of rocks with three distinct 

 velocities. The aprons of detritus which have been deposited on and around con- 

 tinental margins and in marginal geosynclines result in a very complex depositional 

 picture. Progressive orogenies have in many cases further complicated an already 

 intricate stratigraphy. Luckily modern advances in marine geophysics offer some 

 hope that the tangle can some day be unravelled. 



THE ATLANTIC AND GULF TERRACES 



The Atlantic and Gulf terraces have had a parallel evolution, and, when considering 

 their development, the coastal plain and continental platform should be taken as a 

 unit. In the case of the Atlantic the oldest beds are Lower Cretaceous, and in the 

 Gulf, Jurassic. In simplest terms both are comprised of wedges of inter-fingering 

 continental and marine sediments which thicken in a seaward direction, and in both 

 instances deposition is still going on in the same fashion (Fig. 2). In the Gulf Coastal 

 Plain the formations may be considered a series of truncated wedges lying one on the 

 other, and normal faulting that is contemporaneous with deposition is common 

 (Storm, 1945). This fact should be remembered when the peculiar topography of 

 the Gulf slope is discussed, because the major faults have been traced to the limit of 

 drilhng, and furthermore the displacement increases downwards, amounting to 

 hundreds of feet in the older beds, in contrast to but a few near the surface. No such 

 extensive faulting occurs in the Atlantic Plain, although gentle archings and down- 

 warps occur due to basement topography, as is also the case with the Gulf. Both of 

 these terraces have been built unconformably on old erosion surfaces. Updip. where 

 the basement is known, in the western section, the sediments of the Gulf Plain rest 

 on the deformed Palaeozoic rocks of the Ouachita system, and in its eastern portions 

 the Cretaceous laps over the southwestward plunging Appalachians (King. 1951); 

 downdip the formations have thickened so much that the drill has not reached those 

 rocks or any others which may form the basement in a seaward direction. The surface 

 on which the Atlantic Plain rests is well known throughout due to lesser accumulation. 

 The formations feather out updip against the crystallines of the Fall Line, and a crys- 

 talHne igneous basement has been reached many times in drilling for oil or water 

 over its whole extent (Spangler and Peterson. 1950; Spangler, 1950). It has 

 every appearance of being a peneplaned surface which at present has a gentle seaward 

 dip increasing towards the coastline. 



The stratigraphic column at the present shoreline in the Gulf is many times thicker 

 than it is on the Atlantic side; the seas advanced much farther inland and the plains 



