GULF OF MEXICO 



57 



well-developed and somewhat controversial shore- 

 lines are reported from manj- places in the interval 

 from about +3 to +10 feet. However, carefully 

 selected, stable, protected, inner shoreline sectors 

 on North Pacific Islands (Stearns 1941, 1945) and 

 in Australia (Fairbridge 1948) exhibit shoreline 

 cliffs in even-grained limestones with solution 

 notches at about +3, +5 and +8 feet. These 

 seem to represent worldwide stillstands of the 

 sea (eustatic shorelines). Shorelines reported at 

 + 16 and +20 feet (Daly 1934 and others), are 

 not as yet substantiated by data of unquestioned 

 accuracy. 



In places around the shores of the Gulf, there 

 are definite indications of shoreline terraces that 

 seem to indicate stillstands at about +5 and +8 

 feet. An elevated barrier island and coastal 

 lagoon caught by the 10-foot contour has been 

 mapped in Florida by MacNeil (1950) as the 

 "Silver BluflF shoreline" (Parker and Cooke, 1944, 

 pi. 4, fig. B). He did not follow it across southern 

 Florida or on the west side of the peninsula. 



Low shoreline flats appear in many places around 

 the Gulf, but have not been critically studied in 

 the field. Such a low bench shows in air photo- 

 graphs along the base of the high bluffs of the 

 Champoton-Campeche limestone fault-block sali- 

 ent (fig. 12; Sector 2.2, fig. 14). It seems to have 

 a gray, sandy soil. A flat along the front of the 

 elevated Ingleside shoreline between the Rio 

 Grande and Brazos-Colorado deltas (fig. 12) lying 

 at irom about 1 to about 5 feet above mean sea 

 level has low, subdued spits and bars on its sur- 

 face '* and seems to be an emergent marine plain. 

 It is about 0.3 mile wide. This flat may be a 

 nondeltaic part of the original Pleistocene surface 

 in front of this barrier. Deltaic deposits appear 

 along the Gulf side of this barrier east of Galveston 

 Bay. 



Marsh borders the Pleistocene delta of Brazos 

 River in Texas to an elevation of 2 to 3 feet above 

 mean sea level. Just behind the marsh is a bench 

 1.0 to 1.5 mile wide at 3.0 to 4.5 feet with a low 

 nip or wall between 4.0 and 6.5 feet above sea. 

 This bench may be a low Silver Bluff representa- 

 tive. 



At Buhler, a few miles northwest of Lake 

 Charles, Louisiana, the Ingleside barrier and 

 lagoon clays are well preserved. The top-of-clays, 



" Obscured by mima (pimple) mounds higher and wider than the spits 

 (Price 1949). 



representing the approximate shoreline position, 

 lies between 22 and 25 feet above sea. This shore- 

 line and the associated features are well defined 

 running at the same elevation from near Lake 

 Charles west to Beaumont and thence southwest 

 through Fannette, Jefferson County, Texas. 

 Where the shoreline comes within about 10 miles 

 of Anahauc, Chambers County, it is sloping down 

 to the southwest at about 1.5 feet per mile and 

 reaches sea level at Smith Point on the shore of 

 Galveston Bay. Before the formation of the bay, 

 it was formerly tied there to the Brazos delta. 

 The Ingleside shoreline seems to correlate with 

 the Pamlico through the emergent barrier of 

 Gulfport and Biloxi, Mississippi. 



The deltaic plain lying south of the Ingleside in 

 southwestern Louisiana and in Jefferson and 

 Chambers counties, Texas, is of the same age as 

 that immediately  to the north of it, Prairie or 

 Beaumont (Hayes and Kennedy 1903, pp. 27-38; 

 Deussen 1924, p. 110). Along the shore of Jeffer- 

 son and Chambers counties, it is a partly sub- 

 merged deltaic plain. 



The Ingleside appears again south of the 

 Brazos-Colorado delta along the coastal lagoon 

 that opens from Matagorda Bay at its southeast 

 extremity and runs from there to the north flank 

 of Rio Grande delta, the shoreline (top of clay) 

 being at approximate^ 5 to 10 feet above sea. 



The disagreements in the shoreline data for the 

 northern Gulf coast would be removed if the coast 

 from Florida to the Mississippi delta had been 

 stable since Pamlico time, but a slight amount of 

 gulfward downwarping had occurred between the 

 vicinity of Galveston Bay and the coast of Mexico 

 at some point north of Tampico. 



The post-Pleistocene gulfward downwarp of the 

 Beaumont Pleistocene plain (Doering 1935) in- 

 creases in amount from about 1 foot per mile in 

 southeastern Texas to 2 feet per mile southwest of 

 Matagorda Bay. This downwarp seems to mark 

 the influence of the young orogenic coast of 

 Mexico, which it is approaching.'^ This interpre- 

 tation suggests that the emergent shoreline flat on 

 the Gulfward flank of the Ingleside barrier may be 

 either of Ingleside age, down warped some 15 feet, 

 or a younger post- warping shoreline, possibly of 

 Silver Bluff age. Against a Recent age for the 

 low bench is the seeming absence of marine fossils 



'" Corpus Christi lies 175 miles east of folded Cretaceous rocks at the surface 

 iu Meiico and 125 miles northwest of submerged mountains in the Gulf. 



