GULF OF MEXICO 



61 



Sedimentation and the profiles. — The shoreface, 

 ramp and camber of the normal coastal plain shelf, 

 as exhibited on the Gulf of Mexico, seem to have 

 specific characteristics as to sedimentation (map, 

 fig. 16, p. 79) and erosion. From meager data, it 

 seems that sand and shifting bars characterize the 

 shoreface. Contemporary sands, relict deltas and 

 barriers of former sea levels, with some contemp- 

 orary clay deposition, characterize the ramp. Ex- 

 cept when the entire profile is migrating landward, 

 transportation probably dominates the ramp after 

 any rehct elevations have been removed from the 

 part under consideration. Fine-grained sedi- 

 ments, mostly land-derived clays, and presumably 

 the process of deposition, characterize the camber. 

 Off the mouths of large deltas, little or no coarse 

 sand reaches the Gulf and the charts show "mud" 

 beginning near shore. Where sand is present it 

 usually extends to 5 to 10 fathoms (Bates 1953; 

 Lohse 1952). 



Dietz and Menard (1951) have lately advanced 

 evidence and argument for the belief that, at the 

 level of the passage of the shelf from the steep 

 concavity into the gentler slope, in present ter- 

 minology, where the shoreface joins the ramp, is 

 found the depth of maximum wave action on the 

 bottom. They term it the depth of maximum 

 abrasion, replacing the older concept of "wave 

 base." 



If the Gulf has remained essentially at the same 

 level for the past 3,000 to 5,000 years, as pre- 

 viously suggested, it is evident that, on bottoms 

 closely approximating the hj^perbolic curve the 

 shelf bottom must be in equilibrium. This should 

 be true especially in coastal materials of slight 

 resistance and where large amounts of marine 



energy have been effectively applied. That the 

 topography of the bottom is a simple mathematical 

 surface with a hyperbolic bottom profile, is be- 

 lieved to indicate that the forces are in equilibrium. 

 Wliere the bottoms are of hard rock and largely 

 retain a subaerial topography, it may be concluded 

 that the marine forces have inherited a surface 

 produced under different conditions which they 

 have been unable to destroy or to which they 

 happen to be more or less adjusted. 



The equilibrium profile of the coastal plain shelf 

 is in a state of dynamic, not static, equilibrium. 

 In dynamic equilibrium, variations of temporary, 

 short-term value are to be expected. Thus, 

 heavy storm waves are known to shift offshore 

 bars ^^ temporarily as much as a half-mile from 

 their previous positions on the shoreface. Varia- 

 tion of the equilibrium will be about the mean. 

 Marked departures from the mean are caused 

 only by forces external to those in equUibrium. 

 The shift of a river mouth, the coming of a lava 

 flow, or the warping of the earth's crust, would be 

 external forces or conditions which might upset a 

 previously existing equilibrium on the shelf. 



Usefulness of equilibrium profile. — Despite some 

 pessimism (Kuenen 1950, p. 302) as to the value 

 of the profile in geologic studies and much mis- 

 conception on the part of writers as to the differ- 

 ence between static and dynamic equilibria in 

 nature, the present writer finds that the profile of 

 equilibrium is a suitable index of the response of 

 a continental shelf bottom to the application of 

 marine energy for a significant period of time. 

 If, as some think, there have been several oscilla- 

 tions of sea level of as much as 10 to 20 feet during 

 the past 4,000 years or so, a proposition that re- 

 mains unproved, then the interpretation of the 

 modification of the shorelines and shelf by marine 

 energy is less clear than as here tentatively 

 presented. 



Theoretical breakerless curve fits Florida. — Keu- 

 legan and Krumbein (1949) made a theoretical 

 study of the critical steepest bottom slope in 

 shallow water on a shelf across which waves from 

 deep oceanic waters may move but be constantly 

 deformed and constantly lose energy so that they 

 arrive at and near shore without enough height or 

 energy to break or to develop shore structures, 

 such as beaches or cliffs. The absence of such 



" The true underwater feature, not the barrier island. This occured at 

 Galveston, Teias. 



