50 



FISHERY BULLETIN OF THE FISH AND WILDLIFE SERVICE 



several feet high protrude thi'ough the beach in 

 places (Sapper 1937). Marshy, swampy, and 

 partly mud-filled coastal lagoons lie behind the 

 barriers. They are extensively occupied by man- 

 grove swamp forest. These lagoons are called 

 "rivers" on some maps. They were formerly 

 thought to form a continuous inner waterway 

 across the north end of the Peninsula. 



The short beach-bearing sector in the Campeche 

 coast between the towns of Campeche and 

 Champton (fig. 12), seems from air photographs 

 and ground-elevation figures (20 feet to the north 

 against 400 to 500 feet in the block) to be an 

 uplifted fault block of limestone with entrenched 

 stream valleys floored by narrow alluvial plains. 

 The Gulf ends of these alluvial deposits have 

 sandy-to-cobbly pocket beaches. Observers re- 

 port seeing large blocks of limestone on some of 

 them. One report, probably, erroneous, calls 

 some of these blocks and a nearby outcrop 

 "igneous" rock. 



BIOGENOUS ENVIRONMENT 



Where, on the coasts of the Gulf, land-derived 

 sediments have been and are now scarce, sediments 

 of organic origin with large marine organic struc- 

 tures become conspicuous. Such a biogenous 

 environment (fig. 14, Sector 4) (Fleming and 

 Elliott 1950) may vary, here and there, from a 

 brackish lagoonal and inshore enviroimient to a 

 marine environment with waters of normal salinity 

 or salinities somewhat above average (Trask 

 1937) . Where the water is now, or has lately been, 

 warm, tropical and of at least normal marine 

 salinity, coral reefs thrive. The physical limita- 

 tions of this environment have been long and 

 widely discussed. 



The biogenous environment is an oceano- 

 graphic condition existing as an overlay on the 

 basic geological coastal structures. It may occur 

 on any type of shoreline where, and so long as, 

 the requisite sedimentary and oceanographic 

 conditions previously mentioned occur. The biog- 

 enous environment includes the carbonate en- 

 vironment, where MoUusca and corals are con- 

 spicuous among the sedentary organisms, and the 

 paralic or marine swamp and marsh environ- 

 ments, such as those of the mangrove and salt- 

 water grasses and reeds. 



It may be that, with further analysis, a funda- 

 mental geological coastal type of biogenous 



nature may be recognized. Thus, the limestone 

 peninsulas of Florida and Yucatdn may, from the 

 historical point of view, be considered geologically 

 biogenous, since the limestones have been built 

 up for millions of years under dominantly cal- 

 careous biogenous conditions. The Cuban coast, 

 and the Gulf coast of Mexico west of the Yucatdn 

 Peninsula, are today only superficially biogenous, 

 as the organic growths and sediments form a mere 

 patchwork skin on the rock folds. Limestone 

 series several thousands of feet thick among the 

 folded and faulted rocks of Cuba, however, show 

 that the site of the island was biogenous for 

 millions of years. Deposits of argillaceous (clayey) 

 shales and the great earth-deforming (tectonic) 

 events, were major interruptions in the carbonate 

 type of biogenous environment in Cuba. The 

 structural conditions of Cuba today overshadow, 

 for geologists, the biogenous history. 



Carbonate subdivisional environment. — Subsec- 

 tors of the biogenous coasts (Sectors 4) present 

 a variety of structures and bottom types. Coral 

 reefs and the carbonate environment in general 

 occur on both broad (fig. 14, 4.1) and narrow (4.2) 

 shelves. Large shelf areas have a conspicuous 

 bottom-dwelling population. Among these, 

 sponges are conspicuous. Actively growing coral 

 reefs (Smith, p. 292) include fringing and barrier 

 reefs on Cuba and a barrier reef along the outer 

 side of the Florida Keys. This coral barrier runs 

 along the edge of the shelf facing the Straits of 

 Florida at the far southern end of the peninsula. 

 Fringing reefs are also found here and there on 

 other coastal sectors, as near the mouths of 

 streams on the Mexican coast (Sectors 1.11, 3.1, 

 3.2) and on 4.1 on the Yucatan Peninsula. The 

 great Colorados Barrier Reef of northwestern Cuba 

 is fringing at its eastern end but encloses a 15-mile- 

 wide lagoon to the west. 



Atolls and atoll-like coral reefs of more or less 

 tabular form occur west of the Florida Keys 

 (Dry Tortugas atoll) and others form a great, dis- 

 continuous, barrier range along the northern and 

 northwestern margins of the Yucatdn shelf, called 

 the Campeche Banks (Smith, fig. 62, p. 292). The 

 best known of these is the large Alacran atoll. 

 The Marquesas detrital atoll off Florida (Vaughan 

 1914; Cooke 1939, fig. 31) is not known to have 

 coralline growth, the reef being a group of sand 

 keys of shell detritus formed on the shelf by the 

 strong westward currents and winds. The Mar- 



