ANTILLEAN-CARIBBEAN REGION 



879 



Passage, where the water ranges from 1200 to 3760 feet in depth. 



The central part of Puerto Rico is a rugged, east-west-trending moun- 

 tainous mass of the basement complex rocks, and averages about 2000 

 feet in height. 



A coastal plain is particularly prominent along the north side, and a 

 more limited one occurs along the south side. These have been studied 

 in detail by Zapp et al. (1948). A rugged foothills belt flanks the south 

 side of the central Cordillera. 



Geology. Kaye (1957) notes two major structural and stratigraphic 

 rock units in Puerto Rico: the older complex, ranging in known age from 

 Late Cretaceous to late Paleocene or early Eocene, and the middle 

 Tertiary sequence, ranging from late Oligocene possibly to late Miocene. 

 The former rocks are eugeosynclinal in character and the latter non- 

 volcanic, made up dominantly of calcareous marine sediments. The middle 

 Tertiary crops out on the north and south sides of the island and in struc- 

 tural troughs on the west coast. On the north coast the beds dip gently 

 to the north, and, except for slight terracings and a flexure at the north- 

 western corner of the island, are not folded. The middle Tertiary sequence 

 on the south side of the island is somewhat folded. Seismic-reflection 

 studies of the north coast indicate, however, a pronounced northward 

 thickening, possibly some folding, and unconformities at depth. Uncon- 

 formities which may be local have also been noted at several places on 

 the surface. 



Berryhill et al. (1960) have detailed the Upper Cretaceous stratigraphy 

 and facies changes, which may be summarized as follows: 



Rocks of Late Cretaceous age (Turonian to Maestrichitan) in Puerto Rico 

 are of three types: (1) primary volcanic rocks, including tuffs, tuff breccias, 

 and lavas; (2) intermixed pyroclastic and epiclastic rocks, including volcanic 

 conglomerates, volcanic sandstones, and volcanic siltstones; and (3) limestones, 

 most of which were formed as reefs around volcanic islands. These rocks, 

 which have a maximum thickness of more than 20,000 feet, crop out along the 

 crest and flanks of a complexly faulted, northwestward-trending anticlinorium 

 that forms the mountainous core of Puerto Rico. 



The major aspect of the structure of the eastern part of the island is 

 anticlinal which Berryhill et al. believe is due to doming of the strata 

 during intrusion of a batholith in early Tertiary time. See map, Fig. 42.6. 



s j* j j Mm 



MM Ml 



RAHOOIORITC 



obotjl, Early 



Fig. 42.6. Geologic map of Puerto Rico. Compiled from Kaye (1957), Berryhi 

 Glover (I960), and Mattson (1960). Hachured faults indicate graben. 



Briggs and 



Complex faulting that accompanied the batholithic intrusion helped to ac- 

 centuate the anticlinal structure hut in some places modified it. The regional 

 trend of the main faults and also many of the subsidiary faults is west-northwest, 

 but some of the subsidiary faults diverge from that general pattern. 



Two faults of regional significance are recognized in eastern Puerto Rico. 

 One crosses die northern part of the island, and the other traverses the south- 

 central part. 



Movement along the northern fault appears to have been largely transcurrent. 

 The crustal block north of the fault apparently has moved eastward relative to 

 the block south of the fault. 



The subsidiarv, northwestward-trending faults on the north formed as tears 

 along the main fault. Movement along most of these subsidiary faults appears 

 to have been both horizontal and vertical because of rotational movement <>f 

 the blocks formed by the faults. Associated with the northern fault are two 

 grabens. ... A third, smaller graben, . . . lies south of the northern fault. The 

 stratigraphic displacement at the southeastern end of the largest of these three 

 grabens is approximately 6,000 feet. 



The second regional fault, which trends west-northwestward across the 

 southern part of the island, appears to be in part a transcurrent fault and in 

 part a high-angle reverse fault which dips about 70° toward the southwest 

 The stratigraphic displacement along this fault, based on good stratigraphic 

 control is about 12.000 feet. 



The pattern of faulting is related to the crude west-northwestward alignment 



