48 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



From detailed topographic data it is estimated 

 that about one-fifth of the wind-blown clay ex- 

 cavated from playa lake basins is caught on the 

 dunes as sand-sized pellets and the remainder 

 passes inland as dust. The saline plain is nar- 

 rower in Willacy County to the north where the 

 Recent delta and the zone of playas and dunes is 

 narrower than to the south near the Rio Grande. 



Unentrenched deltaic sector. — Sector 1.11, the 

 coast of Tabasco and parts of Veracruz and Cam- 

 peche, Mexico (figs. 12, 14), is a simple deltaic 

 coast with a tropical rain-forest and fairly wide 

 tidal streams that are not embayed. The large 

 Laguna de Terminos is a delta-margin depression, 

 a feature which Bates (1953) thinks is normally a 

 nondepositional basin. Sinking by compaction, 

 former entrenchment and enlargement by wave 

 and current scour are factors that aid in shaping 

 some of the delta-margin basins. This sector has 

 a broad, gentle deltaic plain, abundantlj' crossed 

 by innumerable courses of the Tonala, Seco, 

 Grijalva, Teapao, Usumacinta, San Pedro Y San 

 Pablo, and Palizada Rivers. These courses are 

 grouped into two main deltas; the Seco-Grijalva 

 delta at the west, with a broadly and symmetrically 

 bowed shorehne, and the asymmetrically bowed 

 Grijalva-San Pedro Y San Pablo delta at the east. 

 The latter has a small cuspate mouth. 



DROWNED LIMESTONE-PLATEAU COASTAL 

 PLAINS 



Continental or insular shelves may exist off the 

 above-water parts of oceanic shoals appearing as 

 island groups or as peninsulas attached to coa- 

 tinents. Very broad shelves, upwards of 100 miles 

 wide, border the peninsulas of Florida and Yuca- 

 tan in the Gulf (fig. 13 and No. 2 Sectors, fig. 14). 

 These low peninsulas are great uplifted limestone 

 shoals, now partly drowned limestone plateaus. 

 Their origins have been discussed elsewhere (Price 

 1951b). The surfaces of these plateaus, both 

 above and below water, show a young rolling 

 karst topography of limestone solution with 

 solution-basins and sinkholes. Surface drainage 

 is locally absent and is supplemented by under- 

 ground water circulation moving tlirough solution 

 channels. The Florida limestone is abundantly 

 fissured, at least at the northwest (Vernon 1951). 



The plateau peninsulas are terraced limestone 

 coastal plains. They have delivered a minimum 

 of land-derived detrital sediment to the shelves, 



so that, under tropical climates, these shelves 

 in places abound and probably have long so 

 abounded in great coral reefs (F. G. W. Smith, 

 p. 291) and some reef-like bars and sand keys 

 of shell detritus. 



The sinkhole topography of the limestone 

 plateaus is of subaerial origin, now modified in a 

 broad belt near the shoreline, both above and 

 below water, by coastal deposits (Vernon 1951)'^ 

 and an undetermined amount of solutional activ- 

 ity (Fairbridge 1948). There are a few relatively 

 narrow, submerged stream valleys. Submerged 

 subaerial karst basins are, so far as known, only 

 shallowly filled with a foot or two of sediment, 

 yielding poor anchorage for ships. Offshore bot- 

 tom slopes of the inner half or more of the conti- 

 nental shelf are very gentle (fig. 15, curve 6) to 

 moderately gentle (fig. 15, curve 4), ranging from 

 about 1.5 to 2.5 feet per statute mile. For a few 

 miles offshore, there are many, irregular, shifting 

 bars of shelly sand. 



The limestone-plateau coasts have three types 

 of subsectors: slightly elevated drowned karst 

 salients of a low marshy coast (2.1), beach-bor- 

 dered (2.2), and mangrove-ridge (4.1) shorelines. 

 These show shoreline modification and smoothing 

 ranging from a virtual zero modification through 

 incipient planation to nearly completely smooth 

 beach-bordered coasts. Coastal marsh and swamp 

 of the limestone plateaus are abundantly chan- 

 neled perpendicular to the shoreline by tidal 

 scour. The tides are higher on the peninsula 

 coast of Florida (range 2 to 4.5 feet) than on any 

 other part of the Gulf shoreline. Inshore on the 

 drowned karst coast, and offshore on it and on 

 the other subsectors of the limestone plateaus, 

 we have the so-called carbonate environment of 

 the continental shelf (Trask 1937). 



DROWNED KARST SHORELINE SUBSECTOR 



Subsector 2.1 (fig. 14), along the northern coast 

 of peninsular Florida north of Anclote Keys, near 

 Tampa, has a new type, the drowned karst shore- 

 line. Short convex areas have an intricate, cren- 

 ulate shoreline with many small shoreline basins 

 and archipelagoes of stony islets. Much of this 

 karst shows, on the scale of the navigation charts, 

 no modification by marine agencies. This entire 

 subsector lacks embayed drowned stream valleys 



" Zones of submerged bars and their uplifted counterparts on elevated 

 terraces. 



