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Peter R. Briere |
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Regional Setting Puerto Rico, the easternmost island of the Greater Antilles, is roughly 56 kilometers (km) north to south and 160 km east to west. The island, situated within the seismically active Caribbean-North American plate boundary zone (figure 1) is bounded by the Puerto Rico Trench on the north, Muertos Trough on the south, Virgin Islands Platform on the east, and Mona Passage on the west. The obliquely-convergent Caribbean-North American plate boundary zone is active at a rate of about 37 mm/year (McCann, 1984). The island can be divided into three main geographic regions: 1.) a central east-west mountainous area, 2.) a limestone belt in the northwest to north-central area, and 3.) a discontinuous fringe of relatively flat coastal plains in the south and northeast (Monroe, 1980a). The maximum elevations are at 1,077 m in the east (El Toro) and 1,338 m in the west (Cerro de Punta) (figure 1). Correspondingly, the majority of all the island’s rainfall is orographic; moist ocean air is carried inland where it cools over the mountains and condenses in the form of rain (Calvesbert, 1970). Lineaments Lineaments are mappable linear surface features which differ distinctly from the patterns of adjacent features and presumably reflect subsurface phenomena (O’Leary et al. 1976). They are generally manifested by topography (including straight stream segments), vegetation, or soil tonal alignments (Lattman and Parizek, 1964). Lineament mapping and analyses have gained popularity with the increasing availability of satellite and high-altitude aircraft images. The conclusion of Boyer and McQueen (1964) that remotely sensed linear features are largely a reflection of rock fractures, emphasized by vegetation and topography, has been evinced by successful use of lineament analysis in exploration for oil and gas traps, and to select drilling locations for maximum porosity in tight formations (Peterson, 1980; Mah et al. 1995). Also, hydrogeologists are successfully using lineaments to locate high-yield wells and describe more accurately subsurface structures important for governing recharge, migration, and discharge of groundwater (Fetter, 1994; Brown, 1994). Objective The major aim of this study is to determine the utility of SLAR imagery for geologic mapping on a tropical island where access on foot is difficult because of dense vegetation, rugged terrain, and poor roads. Also, the use of aerial photographs is limited by ubiquitous cloud cover. Within Puerto Rico two major fault zones and several other significant faults have contributed to tilting, folding, and fracturing of the island’s rocks into hundreds of fault blocks. Many problems of tectonic evolution can be solved through the analysis of such fracture systems (Braun, 1982). Remotely sensed imagery, such as SLAR, provides a unique perspective from which to observe these fractures and their relationships to nearby geomorphic features. SIDE-LOOKING AIRBORNE RADAR (SLAR) SLAR images are acquired by sending a beam of radar energy to the ground at an angle perpendicular to the aircraft’s flight path (http://edcwww.cr.usgs.gov/glis/hyper/guide/slar). With SLAR the terrain is illuminated at an oblique angle to enhance subtle geologic structures such as folds and faults. Light and dark areas on the image are caused by high and low radar reflectivity, respectively. The SLAR images of Puerto Rico were captured on May 21 and 23, 1987, at a flight altitude of 8,230 meters above mean sea level, on 46 km-wide strips flown along 20 km flight lines. The STAR-1 Synthetic Aperture Radar System was used in wide swath mode with the look direction east. Sections from the eleven image strips were mosaicked and (figure 2) photogrammetrically controlled to USGS 1:250,000-scale Universal Transverse Mercator (UTM) topographic maps of Puerto Rico. Lineament interpretation in this study was based on the criteria used by Lattman and Parizek (1964), and described by O'Leary et al., 1976. The lineaments were mapped on overlays from individual SLAR swaths, accompanied by a 1:240,000 SLAR mosaic. This overlay was then digitized and imported into ARC/INFO 7.2.1™ software for editing. ArcView 3.0b™ software was used to create the lineament map displayed in figure 4. A total of 636 lineaments were identified, ranging in length from .2 km to 25 km, with a mean of 2.8 km. This totals 1,808 km of lineaments mapped in Puerto Rico. A similar approach was used to create the geologic map in figure 3. The geologic unit boundaries apparent on the SLAR swaths were mapped and then compared to previous geologic maps of Puerto Rico at various scales. A lithologic class was assigned to each mapped unit based on a review of the previously published maps. A total of 14 units were interpreted and used in creating the geologic map in figure 3. A unit boundary mapped from the SLAR often corresponded well to a unit found on a previously published map, but not always the same map. Geologic mapping using the SLAR imagery of Puerto Rico results in a map that contains a group of units not found on any one previously published geologic map of Puerto Rico. GEOLOGIC SETTING The three principal geographic divisions of Puerto Rico include the northwest to northcentral limestone belt, central east-west mountainous area, and the southern and northeastern fringe of relatively flat coastal plains (Monroe, 1980a). These regions have distinctive relief and landform characteristics, and other significant yet smaller attributes. The late Oligocene to Middle Miocene northern limestone belt extends about 135 km, from Rio Grande de Loiza to the west coast, and reaches a maximum width of about 23 km south of Arecibo (Monroe, 1980b). In the east, most of the rocks are buried beneath a series of alluvial deposits. West from San Juan the belt is interrupted only by the wide alluvial valleys (Monroe, 1980b). Drainage of the limestone belt often occurs underground, mostly through thin, interconnected passageways. Conspicuous features in the karst area include the Lares cuesta scarp that extends from San Sebastian to Corozal (figure 1); a discontinuous belt of conical hills (cone karst) north of the Lares scarp; caves; steep-sided hills (mogotes) that rise out of the surrounding plains covered by blanket sands; walls (ramparts) at the tops of the canyon walls and fault scarps; and zanjones (vertical-walled trenches) some more than 1,000 m long (Monroe, 1980b). The northern coastal plains are bordered to the north by sand beaches that pass landward into broad sand and clay flats (Monroe, 1980a). Alluvial fans dominate the southern coastal plains and consist of poorly sorted clastic debris from mountains to the north (Monroe, 1980a). The central highlands in Puerto Rico include exposures of Cretaceous and lower Tertiary volcanic and sedimentary rock sequences, various intrusive rock bodies, Oligocene and Miocene sediments, large areas of floodplain deposits, terrace deposits, and landslide debris (Monroe, 1980b). Volcanism associated with active northwest-striking faults from Cenomanian to Eocene time deposited volcanic rocks (Baweic, in press). Plutons of tonalite and granodiorite composition intruded these rocks from the Aptian to Maastrictian. The Utuado (west-central) and San Lorenzo (east) batholiths were formed during Maastrictian time, which was followed by uplift and erosion. Lineaments and Lithology Geologic features and boundaries interpreted from the SLAR images (figure 3) can be mapped with a level of detail and frequency of mappable features that rivals larger scale field mapping. While SLAR cannot match the detail obtained by onsite field mapping, it does enhance some features not as easily apparent from the ground, such as lineaments. Central Mountainous Region The most striking lineaments visible in the SLAR images are long northwest-trending ones that cross the southern two-thirds of the island (figure 4). They represent the surface expression of the Great Southern Puerto Rico Fault Zone (Scanlon and Southworth, 1989). The predominantly volcanic/plutonic rocks in the central Cordillera exhibit by far the most lineaments. These lineaments are representative of the fracturing resulting from local east-west faulting and the more regional uplift between the Puerto Rico Trench and Muertos Trough. In some places fault blocks can be discerned, particularly in the Utuado Batholith and just north of central Aguirre. In contrast, the San Lorenzo Batholith in the southeast has few lineaments. The lineaments that do appear in this batholith are generally north-south and occasionally east-west. This lineament pattern is different than that of the surrounding terrain and may represent fracturing caused by the cooling of intrusive rocks. The long east-west scarp separating the Tertiary carbonate rocks of northern Puerto Rico from the mountainous central core of volcanic rocks and intrusive granodiorites of Cretaceous and early Tertiary age (Briggs, 1964) is clearly visible in the mosaic. Limestone Belt Some features visible on the SLAR images of Puerto Rico include cone karst, sinks, mogotes, and zanjones. These landforms occur mainly in the northern limestone belt, primarily representations of zanjones or mogotes in the northwest. Cone karst is common in tropical latitudes (Monroe, 1980b). Sinuous depressions surrounding conical hills are the cone karst signature (Monroe, 1980b). Origins of these hills have been argued, and they may reflect cavern collapse of underground streams, joint control, or erosion by streams that cut down into the limestone (Monroe, 1980b). Aptly described by Monroe (1980b) as a "lunarlike landscape" created by thousands of sinks, these conical hills are visible in the limestone areas, as are mogotes, the steep-sided hills particularly apparent in the southwestern regions of the SLAR. Zanjones, the vertical-walled, parallel trenches created from joint enlargement by limestone solution, are found in northwest Puerto Rico. They occur in the generally flat areas of strongly stratified limestone (Monroe, 1980b). In contrast to the central mountainous area, many lineaments in the limestone belt trend east-west without a northwest component. Some even trend southwest-northeast. Many of these lineaments may be the surface expression of hard, thick-bedded massive and dense limestone, and limestone with calcarenite. Coastal Plains The northeast surficial deposits are virtually devoid of lineaments, with a few straight stream segments the exceptions. An example is the abnormally straight Rio Grande de Loiza which flows its last 4 km without even a minor bend. The southern coastal sediments, like their northern counterparts, display few lineaments, because thick recent sediment obscures basement features. Although the 45 km-long fault postulated by Briggs (1964) was not seen between Arecibo and Barceloneta, other evidence for local fault movements was derived from the northern fluvial systems. Within the region including Rio Guajataca, Rio Camuy, and Rio Grande de Arecibo, correlating past tectonic events may be possible. Rio Guajataca and Rio Camuy display strong similarities characterized by sudden course changes and changes in stream morphometry. These systems may give clues to local and regional tectonics not apparent in the surrounding limestone. Lineament frequency and occurrence derived from the SLAR swaths can be explained by the surrounding geology, and can help clarify some of the geologic questions. In areas with thick surficial cover, for instance, there are few lineaments, but those lineaments that are apparent from fluvial systems or karst topography may help explain some of the tectonic history covered by the sediments. The geologic map based on the SLAR correlates well with geologic maps produced by onsite field mapping and compiled at larger scales. Some map units that were clearly visible on the SLAR were omitted on some geologic maps, but included on others. Other mappable features (such as lineaments) on the SLAR are less readily apparent from the field. Also, in many instances the use of SLAR imagery could be more efficient than mosaicking and interpreting a large quantity of aerial photographs. The utility of SLAR for geologic mapping in Puerto Rico lies in the ease with which both structure and lithology are discernable. Unlike visible and near-infrared wavelengths, radar energy (at the correct wavelength) can penetrate most clouds, making it an especially useful tool in Puerto Rico where a persistent cloud cover generally obscures parts of the island. SLAR can also be used to supplement other forms of data, such as aerial photographs and satellite imagery, which are limited by cloud cover. This study of Puerto Rico suggests that SLAR would be useful on other tropical islands with the same difficult conditions (vegetation, rugged terrain, cloud cover, and poor roads).
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