Douglas L. Smith

Origin of the epeirogenic uplift of Pliocene-Pleistocene beach ridges in Florida and development of the Florida karst

Marine fossils of Pleistocene age are known to occur in beach ridges near the border of northern Florida and southern Georgia at elevations of between 42 and 49 m above mean sea level. No evidence exists for a massive melt-off of glacial ice, which would be required to raise sea level to these elevations. Florida, therefore, must have been uplifted epeirogenically during the Pleistocene. Measurement of dissolved solids in Florida9s springs demonstrates that the karst area is losing a minimum of 1.2 × 10 6 m 3 /yr of limestone through spring flow, the equivalent of 1 m of surficial limestone every 38,000 yr. This loss has led to an isostatic uplift of the north-central part of the Florida peninsula of at least 36 m during Pleistocene and Holocene time, which agrees with observed elevations of marine terraces.

Review of the tectonic history of the Florida basement

Abstract Lithological similarities between the Suwannee Basin deposits of undeformed Ordovician and Silurian sandstone and shale lying 0.9–1.9 km below the north Florida surface and equivalent-aged strata from Senegal to Sierra Leone in western Africa suggest an African location for the Florida basement during the early Paleozoic. The southern edge of the Paleozoic sediments laps onto a Pan African granitoid batholith which is considered representative of an early Paleozoic Afro-South American assemblage. A late Paleozoic (Allegheny) continental closure brought the Florida basement, and much of the present-day Gulf Coastal margin, into juxtaposition with North America, but the arrangement of continental promontories precluded a direct application of deforming stresses to the overlying basin sediments. Numerous occurrences of early Mesozoic rhyolitic tuffs and ignimbrites in deep (2.7–5.5 km) bore holes from central and south Florida are interpreted as indicative of a Triassic hot spot that initiated rifting and the opening of the North Atlantic. Subsurface rhyolitic rocks are also present in northern Florida, but the position of the hot spot dictated that the Florida Plateau remain appended to North America as radially propagating rifts from the hot spot created a new configuration of plate boundaries from those of the Paleozoic. Other continental terrains adjacent to southern Florida were either altered and foundered to underlie the Bahamas or stayed with Africa and/or South America.

Florida as an exotic terrane: Paleomagnetic and geochronologic investigation of lower Paleozoic rocks from the subsurface of Florida

New geophysical and geochronological data are presented for a lower Paleozoic subsurface (794–827 m depth) sedimentary core recovered from Alachua County, northern Florida. Paleomagnetic data from samples taken at various depths along the core indicate that a high unblocking temperature component is isolated during thermal demagnetization between 400 and 600 °C. The mean inclination of this component is 65°, yielding a paleolatitude of 49°. Previous paleogeographic reconstructions for the early Paleozoic of North America place Florida at a significantly lower paleolatitude (∼28°), whereas our data are consistent with a Gondwanian fit (∼50°–60°). U/Pb isotopic systematics of zircons separated from core samples yield five dates ranging from ca. 1650 to 1800 Ma. Because there are no known source rocks of this antiquity in southeastern North America, the most probable provenance for these detrital zircons is either Africa or South America. These two new lines of geologic data provide strong evidence confirming previous suggestions that Florida was part of Gondwana during the early Paleozoic and that its current configuration is that of an exotic terrane sutured to North America during the fragmentation of Pangea.

Caribbean Plate Relative Motions

Any reconstruction of the geological history of the Central America land bridge is dependent on conjectural motions of several lithospheric plates. The most important aspect of a timing sequence for a link between the North and South America land masses is apparently the creation of the Caribbean Plate. The geological record, however, which details a tectonic evolution of the Caribbean Plate and its margins is so enigmatic that many critical questions remain unresolved, and every developed model is couched in some incompatibilities.

Heat flow, radioactive heat generation, and theoretical tectonics for northwestern Mexico

Abstract Nine heat flow values from Baja California and Sonora represent the first determinations of heat flux from Mexico and, in general, justify southerly extensions of the Basin and Range and Southern California thermal provinces of the southwestern United States. Heat flow in northern Baja California is low (0.84 hfu), consistent with known values from the Sierra Nevada Batholith, but a value of 1.23 hfu was recorded in the central area of the peninsula. Seven measurements from Sonora, averaging 2.5 hfu, are similar to Basin and Range values. Reduced heat flow values ( q* ) have been calculated for Sonora and are interpreted as indicative of a separate Basin and Range thermal subprovince in Mexico. A tectonic model developed for northwestern Mexico attributes the high heat flow of the Basin and Range to behind arc extension associated with the termination of Cenozoic subduction. Opening of the Gulf of California and sea-floor spreading therein has provided a vent for the release of heat accumulated under northwestern Mexico. Thus, the southern portion of the Basin and Range province (in Mexico) appears to be cooling and narrowing and a subprovince of high reduced heat flow is defined east and perhaps southwest of the Gulf of California.

Electrical resistivity investigation of potential cavities underlying a proposed ash disposal area

Measurements of electrical resistivity anomalies using a Wenner electrode array in a profiling mode were conducted over a 105-acre area near Crystal River, Florida Analyzed results of collected data were used to determine the likelihood of subsurface solution cavities beneath a proposed ash disposal area “Sounding” measurements with electrode spacing values extending up to 100 feet and Lee electrode measurements were used to identify and locate specific anomaly maxima identified anomalies on the order of 200 chms-feet contrasted with expected values of 300 to 400 ohms-feet and were attributed to lower resistivities inherent in water-filled solution cavities below a shallow water table Prominent cavity complexes at 5 to 15 feet depth were predicted in areas of low resistivity anomalies in the otherwise geoelectrically uniform, Eocene Ocala limestone Shallow test borings confirmed the presence of a significant concentration of shallow cavities in the predicted areas This study demonstrated the feasibility of electrical resistivity measurements as a reconnaissance tool to economically determine subsurface cavities and to aid in developmental and environmental planning

Deformation and patterns of sedimentation, South San Clemente Basin, California Borderland

Abstract Seismic reflection profiling in the South San Clemente Basin and the southern portion of the San Diego Trough has revealed at least six sedimentary units exhibiting varying stages of deformation. Four of the units are interpreted to be marine turbidites supplied by adjacent submarine canyons. Sediments comprising the Descanso Plain and correlative material within the South San Clemente Basin are attributed to a southerly source (Banda Canyon), while the more recent Quaternary turbidites from Coronado Canyon filled the southern San Diego Trough and then spilled over into South San Clemente Basin. The relatively high but intermittent rates of sedimentation, together with shifting sources and areas of deposition, have resulted in sedimentary units that were emplaced in comparatively short episodes but which were subjected to relatively continuous tectonic activity. Consequently, the sedimentary layers of each unit appear uniformly affected by deformation which increases in successively older units.

Heat flow at the Platanares, Honduras, geothermal site

Meert J.G. and Smith, D.L., 1991. Heat flow at the Platanares, Honduras, geothermal site. In: F.Goff (Editor), Honduras - A Geothermal Investigation. J. Volcanol. Geotherm. Res., 45:91-99. Three boreholes, PLTG-1, PLTG-2 and PLTG-3, were drilled in the Platanares, Honduras geothermal system to evaluate the geothermal energy potential of the site. The maximum reservoir temperature was previously estimated at 225 - 240°C using various types of chemical and isotopic geothermometry. Geothermal gradients of 139-239°C/km, calculated from two segments of the temperature-depth profile for borehole PLTG-2, were used to project a minimum depth to the geothermal reservoir of 1.2- 1.7 km. Borehole PLTG-1 exhibited an erratic temperature distribution attributed to fluid movement through a series of isolated horizontal and subhorizontal fractures. The maximum measured temperature in borehole PLTG-1 was 150.4°C, and in PLTG-2 the maximum measured temperature was 104.3°C. PLTG-3 was drilled after this study and the maximum recorded temperature of 165°C is similar to the temperature encountered in PLTG-1. Heat flow values of 392 mWm -2 and 266 mWm -2 represent the first directly-measured heat flow values for Honduras and northen Central America. Radioactive heat generation, based on gamma-ray analyses of uranium, thorium and potassium in five core samples, is less than 2.0 #Wm -3 and does not appear to be a major source of the high heat flow. Several authors have proposed a variety of extensional tectonic environments for western Honduras and these heat flow values, along with published estimates of heat flow, are supportive of this type of tectonic regime.

Heat flow in the Ozark Plateau, Arkansas and Missouri: relationship to groundwater flow

Abstract Heat flow values were calculated from direct measurements of temperature and thermal conductivity at thirteen sites in the Arkansas-Missouri Ozark Plateau region. These thirteen values are augmented by 101 estimates of heat flow, based on thermal conductivity measurements and temperature gradients extrapolated from bottom-hole temperatures. The regional heat flow profile ranges from 9 mW m −2 to over 80 mW m −2 , but at least two distinct thermal regimes have been identified. Seven new heat flow determinations are combined with three previously published values for the St. Francois Mountains (SFM), a Precambrian exposure of granitic and rhyolitic basement rocks, average 47 mW m −2 . Radioactive heat production of 76 samples of the exposed rocks in the SFM averages 2.4 μW m −2 and a typical continental basement contribution of 14 mW m −2 is implied. Conversely, the sedimentary rock sequence of the plateau is characterized by an anomalously low heat flow, averaging approximately 27 mW m −2 . Groundwater transmissivity values that are based on data from 153 wells in deep regional aquifers demonstrate an inverse relationship to the observed heat flow patterns. The areas of high transmissivity that correspond to areas of low total heat flux suggest that the non-conservative vertical heat flow within the Ozark sedimentary sequence can be attributed to the effects of groundwater flow.

Indicators of low-temperature geothermal resources in northern Louisiana and central Mississippi

Measurements of heat flow and near-surface (< 500 m) geothermal gradients in the Gulf Coastal Plain suggest a zone of low-grade geothermal resources extending from northern Louisiana across south-central Mississippi. Subsurface temperatures exceeding 50°C, suitable for space-heating use, seem probable at depths of 1 km. Thermal conditions within the zone are comparable to those known for areas having attractive thermal energy prospects on the Atlantic Coastal Plain.