Geology topics
Research about Puerto Rico
Source-linked reports with geographic coverage including Puerto Rico.
Preliminary report of the geology of the Barranquitas quadrangle, Puerto Rico
No abstract available.
Preliminary report on the geology of the Coamo quadrangle, Puerto Rico
No abstract available.
Geology of the Mayagüez area, Puerto Rico
The Mayagüez area forms the southwestern corner of Puerto Rico , west of 67° W. and south of 18° 15' N. One-third of the 640 square kms is covered by thick alluvium. Unconformities separate a basal complex, two sequences of highly folded igneous and sedimentary rocks, and a younger sequence of gently dipping sedimentary rock. The basal Bermeja complex contains serpentinite, silicified porphyritic volcanic rock with some sedimentary rock, and minor spilite, amphibolitized spilite, and amphibolite. It is exposed chiefly in some anticlinal cores in southwestern Puerto Rico . Limestone, mudstone, andesite, and basalt form the older folded sequence. The Río Loco formation, bronzite andesite porphyry in part with pillow structures, was extruded perhaps in the Cenomanian. The Mayagüez group includes most of the rocks in southwestern Puerto Rico : the Yauco mudstone, Parguera limestone, Brujo limestone, Melones limestone, Maricao basalt, Sabana Grande andesite, and El Rayo volcanic rocks. The maximum possible age range is Turonian to Maestrichtian. The group ranges in thickness from about 800 m in the south to 3800 m in the north, and it varies in lithology from limestone in the south to mudstone and volcanic rock in the north, indicating a volcanic center to the north during that time. The second folded sequence contains andesitic volcanic rock, bedded tuff, and massive limestone. The San Germán formation (Maestrichtian) includes andesite, the Cabo Rojo agglomerate member, and the Cotui limestone member. The Jicara formation, massive limestone and bedded tuff, is Paleocene; there is one exposure of an unnamed ? Eocene marl. Post-Eocene limestone and conglomerate are also exposed in the area . The structure of the basement complex is obscured by its massiveness and by the cover of younger rocks. Two major deformations have affected the rocks of southwestern Puerto Rico since Cenomanian to Santonian time. In the Maestrichtian, the first of these formed folds with a N. 60° W. trend, asymmetric or overturned to the south. Near the south coast the folding of thin Mayagüez group rocks was probably influenced by trends in the Bermeja complex which caused deviations in the regional trends and also some overturning to the north. The San Germán formation, deposited unconformably on the eroded surface of the folded Mayagüez group, contains large allochthonous blocks of older and contemporaneous rocks. These blocks, up to 2 km by 1 km in exposure, were deposited by slumping or sliding due to gravity within and at the base and top of the San Germán formation near Lajas and San Germán. Most rocks in the blocks are extremely contorted and contain deformed Foraminifera. The San Germán and Jicara formations and perhaps the ?Eocene marls were deformed into gentle open folds trending east in the area covered by this report. Oligocene, Miocene, and younger sedimentary rocks have been tilted and uplifted. Large east-west left-lateral transcurrent faults cross the area , offsetting and offset by two sets of transverse faults (N. 45° E., N. 20° W.): most faults are probably Maestrichtian to Oligocene, although minor faulting has continued to the present. Dikes and sills of quartz diorite porphyry and mica-quartz dacite porphyry intrude the ?Maestrichtian San Germán formation and older units. A diorite plug cuts the Bermeja complex, and a granodiorite plug intrudes the Mayagüez group.
Occurrences of alunite, pyrophyllite, and clays in the Cerro La Tiza area, Puerto Rico
A deposit of hydrothermally altered rocks in the Cerro La Tiza area located between the towns of Comerio and Aguas Buenas, approximately 25 kilometers southwest of San Juan, Puerto Rico, was mapped and studied to determine the principal minerals, their extent distribution and origin, and the possibility of their economic utilization, especially in Puerto Rico. The Cerro la Tiza area is about 7 1 / 2 kilometers long, has an average width of about 1 1 / 2 kilometers and embraces a total area of approximately 15 square kilometers. The principal mineralized zone, a dike-like mass of light-colored rocks surrounded by dark-colored volcanic country rocks, occupies the crest and upper slopes of east-trending Cerro La Tiza ridge and is believed to be of Late Cretaceous or Eocene age. This zone is approximately 5,300 meters long, 430 meters wide and has an area of approximately 225 hectares (556 acres). The rocks of the mineralized zone are of mixed character and consist mainly of massive quartzose rocks and banded quartz-alunite rocks closely associated with foliated pyrophyllitic, sericitic and clayey rocks. The principal minerals in probably order of abundance are quartz, alunite, pyrophyllite, kaolin group clays (kaolinite and halloysite) and sericite. Minerals of minor abundance are native sulfure, diaspore, svanbergite (?), sunyite (?), hematite, goethite, pyrite, rutile (?) and very small quantities of unidentified minerals. The mineralized zone has broken down to deposits of earth-rock debris of Quaternary age that cover much of the slopes and flanks of Cerro La Tiza. This debris consists generally of fragments and boulders with a very large size range embedded in a clayey matrix. The distribution of the earth-rock debris with respect to the present topography and drainage suggests that it may have undergone at least two cycles of erosion. Underlying the earth-rock debris and completely enclosing the mineralized zone are country rocks of probably Late Cretaceous age. These consist principally of low flows and volcanic and flow breccias but contain thin interbedded siltstones and sandstones. The lavas are generally predominant at the western end of the area and the breccias at the eastern end. The mineralized zone and the country rocks are sheared along two predominant directions that are approximately N 70 degrees E and N 70 degrees W. The ridge of Cerro La Tiza appears to be a broad shear zone through which hydrothermal emanations gained access to the country rocks. The emanations are believed to have originated from intrusive rocks that probably underlie the area. The surrounding area contains both large and small exposed intrusive bodies. The largest one is the San Lorenzo batholith of Late Cretaceous or Eocene age whose exposed northwest edge is approximately 19 kilometers southeast of the eastern end of the Cerro La Tiza area. Other zones of hydrothermally altered rocks were discovered along a mineralized belt extending eastward from Cerro La Tiza through the Rio Gurabo Valley nearly to the Vieques Passage bordering the east coast of Puerto Rico. Other zones were discovered north and south of this belt and still others were found circumventing the San Lorenzo batholith. The most abundant minerals of the mineralized zone can be exploited for economic utilization in Puerto Rico. Alunite can be utilized in the manufacture of aluminum sulfate for water purification. It can also be used in the manufacture of alumina refractory materials. Pyrophyllite can be used as a carrier for insecticides and fungicides. It can also be utilized for the manufacture of ceramic products, as a filler in the soap industry and as a carrier for paint pigments. Kaolinite can be used in the ceramic industry and in the manufacture of glass as a substitute for feldspar. Halloysite might be utilized as a catalyst support in the cracking of petroleum. Tonnages of reserve ore on Cerro La Tiza are calculated to be 1,590,000 inferred short tons (1,440,000,000 inferred metric tons) of mixed minerals. These tonnages are based on the assumption that the depth of the mineralized zone is one-half of the exposed width. The deposit is well situated for open pit mining, but because of the existing cover of earth-rock debris, soil foliage, exploration should proceed exploitation for better determination of the most promising areas containing the best concentrations of the minerals sought.
Geology of Isla Mona Puerto Rico, and notes on age of Mona Passage
No abstract available.
Notes on the structural geology of Puerto Rico
Two major structural and stratigraphic rock units occur 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 are very badly faulted but for the most part apparently only moderately folded. With the exception of a large, partly low-angle thrust, the writer has not recognized evidence of strong tangential stresses. Intra-formational folding in the older complex is interpreted as caused by submarine sliding (slump structure). Except in the vicinity of the larger plutonic intrusions and in the northeastern and southwestern corners of the island, the main strike alignment of the older complex is northwestward. The plutons are roughtly concordant with the structure of the country rock and show varying degrees of differentiation. Their average composition seems to be more acidic than that of the volcanic and volcanogenetic rocks into which they are intruded. The middle Tertiary sequence is nonvolcanic, made up dominantly of calcareous marine sediments. It crops out on the north and south sides of the island and in structural 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 northwestern 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. Unconformities which may be local have also been noted at several places on the surface. Several large faults in the middle Tertiary sequence have been recognized in both the north- and south-coast belts. The pattern of master joints that is inferred in the north-coast middle Tertiary belt from topographic alignments seems to indicate (1) control by fades contacts, (2) possible downwarping associated with the sinking of an arm of the Puerto Rican Trench, and (3) tension during upwarping along the island axis. The dates of the major diastrophic events that are decipherable from the Puerto Rican data are: (1) early Tertiary (possibly late Paleocene, but more probably Eocene), when the older complex was deformed; (2) Miocene, when the middle Tertiary sequence was deformed; (3) late Pliocene and possibly early Pleistocene, when block faulting on a large scale produced the present topographic relief. Remnants of the fault scarps resulting from the latter deformation occur widely. Quaternary marine deposits and marine terraces suggest that Puerto Rico has been relatively unaffected by crustal movement at least since the late Pleistocene.
Spectrographic analyses of laterite soil from the Mayaguez area, Puerto Rico
No abstract available.
Geology and ground-water resources of Puerto Rico
Puerto Rico, the easternmost and smallest of the Greater Antilles, has an axis of deformed and metamorphosed volcanic rocks of Upper Cretaceous age, intruded by dioritic rocks during the Antillean revolution. The hard-rock core is flanked on the north and south by limestones and clastic rocks of late Oligocene and early Miocene age, which have been gently arched and uplifted. Similar rocks were deposited in late Miocene or early Pliocene time along the west coast. During the Quaternary the island has been separated from the other major Antillean islands by faulting and has been arched, uplifted, and tilted to the northeast. Alluvium and littoral deposits have partially filled the valleys and have formed coastal plains on the north and south. The Upper Cretaceous volcanic and associated rocks yield small supplies of water to wells in most places. The Tertiary limestones yield large supplies in some places on the north, south, and west coasts. The Quaternary sands and gravels are the most important aquifers. They yield about 200 million gallons a day to wells in the main south coastal plain alone. The water is used largely for irrigation. A total of perhaps 250 to 300 million gallons a day is pumped from wells in the island, and moderate to large additional supplies are available in some places.
Sand and gravel resources of Puerto Rico
Many of Puerto Rico's beaches are eroding, and though rates of erosion vary, it is a major concern for the tourism and residential development industries. More than 85 percent of the population lives within 7 kilometers of the coast and they are heavily dependent on tourists that are attracted by the island's beaches and coral reefs. High-quality scientific data are needed to help formulate public policy regarding residential and commercial construction along the coast, beach replenishment, and future use of marine resources. Scientists have long recognized that the causes of coastal land loss are not limited to a relative rise in sea level, but can be manmade as well. For example, sediment supply to beaches especially along the north shore of Puerto Rico has been strongly affected by upstream river channeling, dam construction, various agricultural practices, paving and urbanization, as well as shallow-water oceanographic processes. The response to coastal erosion in Puerto Rico has been mostly crisis based leading to engineered solutions that have a negative effect on the coastal environment.