Kevin J. Cunningham

Impacts of the 2004 tsunami on groundwater resources in Sri Lanka

The 26 December 2004 tsunami caused widespread destruction and contamination of coastal aquifers across southern Asia. Seawater filled domestic open dug wells and also entered the aquifers via direct infiltration during the first flooding waves and later as ponded seawater infiltrated through the permeable sands that are typical of coastal aquifers. In Sri Lanka alone, it is estimated that over 40,000 drinking water wells were either destroyed or contaminated. From February through September 2005, a team of United States, Sri Lankan, and Danish water resource scientists and engineers surveyed the coastal groundwater resources of Sri Lanka to develop an understanding of the impacts of the tsunami and to provide recommendations for the future of coastal water resources in south Asia. In the tsunami-affected areas, seawater was found to have infiltrated and mixed with fresh groundwater lenses as indicated by the elevated groundwater salinity levels. Seawater infiltrated through the shallow vadose zone as well as entered aquifers directly through flooded open wells. Our preliminary transport analysis demonstrates that the intruded seawater has vertically mixed in the aquifers because of both forced and free convection. Widespread pumping of wells to remove seawater was effective in some areas, but overpumping has led to upconing of the saltwater interface and rising salinity. We estimate that groundwater recharge from several monsoon seasons will reduce salinity of many sandy Sri Lankan coastal aquifers. However, the continued sustainability of these small and fragile aquifers for potable water will be difficult because of the rapid growth of human activities that results in more intensive groundwater pumping and increased pollution. Long-term sustainability of coastal aquifers is also impacted by the decrease in sand replenishment of the beaches due to sand mining and erosion.

Prominence of ichnologically influenced macroporosity in the karst Biscayne aquifer: Stratiform "super-K" zones

A combination of cyclostratigraphic, ichnologic, and borehole geophysical analyses of continuous core holes; tracer-test analyses; and lattice Boltzmann fl ow simulations was used to quantify biogenic macroporosity and permeability of the Biscayne aquifer, southeastern Florida. Biogenic macroporosity largely manifests as: (1) ichnogenic macroporosity primarily related to postdepositional burrowing activity by callianassid shrimp and fossilization of components of their complex burrow systems (Ophiomorpha); and (2) biomoldic macroporosity originating from dissolution of fossil hard parts, principally mollusk shells. Ophiomorpha-dominated ichnofabric provides the greatest contribution to hydrologic characteristics in the Biscayne aquifer in a 345 km 2 study area. Stratiform tabular-shaped units of thalassinidean-associated macroporosity are commonly confi ned to the lower part of upward-shallowing highfrequency cycles, throughout aggradational cycles, and, in one case, they stack vertically within the lower part of a high-frequency cycle set. Broad continuity of many of the macroporous units concentrates groundwater fl ow in extremely permeable passageways, thus making the aquifer vulnerable to long-distance transport of contaminants. Ichnogenic macroporosity represents an alternative pathway for concentrated groundwater fl ow that differs considerably from standard karst fl ow-system paradigms, which describe groundwater movement through fractures and cavernous dissolution features. Permeabilities were calculated using lattice Boltzmann methods (LBMs) applied to computer renderings assembled from X-ray computed tomography scans of various biogenic macroporous limestone samples. The highest simulated LBM permeabilities were about fi ve orders of magnitude greater than standard laboratory measurements using air-permeability methods, which are limited in their application to extremely permeable macroporous rock samples. Based on their close conformance to analytical solutions for pipe fl ow, LBMs offer a new means of obtaining accurate permeability values for such materials. We suggest that the stratiform ichnogenic groundwater fl ow zones have permeabilities even more extreme (~2‐5 orders of magnitude higher) than the Jurassic “super-K” zones of the giant Ghawar oil fi eld. The fl ow zones of the Pleistocene Biscayne aquifer provide examples of ichnogenic macroporosity for comparative analysis of origin and evolution in other carbonate aquifers, as well as petroleum reservoirs.

Eustatic implications of late Miocene depositional sequences in the Melilla Basin, northeastern Morocco

Abstract The age (∼5.78 Ma or lower chron C3r) of the major drawdown of the Paleo-Mediterranean Sea during the Messinian Salinity Crisis has been established by combining results from stratigraphy, paleontology, magnetostratigraphy, and argon dating for a late Miocene sedimentary succession in the Melilla Basin, NE Morocco. This event is inferred from a marine-to-continental series of carbonate and siliciclastic rocks that record the end of Messinian marine deposition in the Melilla Basin and presumably marks the final isolation of the Paleo-Mediterranean Sea. The evidence from the Melilla Basin is approximately coeval with an increase in benthic foraminiferal δ 18 O values from a deep-marine section in the Bou Regreg valley, NW Morocco (Hodell et al., 1994). This increase suggests that a glacio-eustatic lowering of sea level, at least, contributed to the final closure of the Mediterranean during the Messinian Salinity Crisis. The marine-to-continental succession onlaps a carbonate complex that contains evidence for multiple relative sea-level changes leading up to the main drawdown. From bottom to top, the carbonate complex is composed of: (1) an onlapping ramp; (2) a prograding bioclastic platform; (3) a prograding and, locally, downstepping Porites -reef complex; and (4) a topography-draping sequence composed of grainstones, Porites reefs, and stromatolites (terminal carbonate complex of Esteban, 1979). The transgressive ramp correlates to relatively low values of benthic foraminiferal δ 18 O values from a Tortonian-to-lower Messinian section at Bou Regreg (Hodell et al., 1994). This correlation indicates, at least in part, a link between rising sea level and a reduction in global ice volume during deposition of the ramp. A major fall in relative sea level (∼60 m) occurred near the demise of the reef complex during chron C3n.1n at 5.95 ± 0.10 Ma. This signals the initiation of drawdown and changing environmental conditions in the Melilla Basin (a marginal basin), and perhaps the entire Paleo-Mediterranean Sea. A megabreccia interpreted as forming by solution collapse of evaporites on the basin margin of the reef complex occurs at the base of the terminal carbonate complex. Updip, a major subaerial unconformity separates the reef complex and terminal carbonate complex. Evaporite deposition likely occurred during this exposure event and has been dated at 5.82 ± 0.02 Ma near the base of chron C3r. We contend that these evaporites, restricted to the shallow Melilla Basin, are related to the continuation of the initial stage of the major drawdown of the Paleo-Mediterranean Sea.

Assessing the vulnerability of a municipal well field to contamination in a karst aquifer

Proposed expansion of extractive lime-rock mines near the Miami–Dade County Northwest well field and Everglades wetland areas has garnered intense scrutiny by government, public, environmental stakeholders, and the media because of concern that mining will increase the risk of pathogen contamination. Rock mines are excavated to the same depth as the well fields primary producing zone. The underlying karst Biscayne aquifer is a triple-porosity system characterized by (1) a matrix of interparticle porosity and separate vug porosity; (2) touching-vug porosity that forms preferred, stratiform passageways; and, less commonly, (3) conduit porosity formed by thin solution pipes, bedding-plane vugs, and cavernous vugs. Existing ground-water flow and particle tracking models do not provide adequate information regarding the ability of the aquifer to limit the advective movement of pathogens and other contaminants. Chemical transport and colloidal mobility properties have been delineated using conservative and microsphere-surrogate tracers for Cryptosporidium parvum . Forced-gradient tests were executed by introducing conservative tracers into injection wells located 100 m (328 ft) from a municipal-supply well. Apparent mean advective velocity between the wells is one to two orders of magnitude greater than previously measured. Touching-vug, stratiform flow zones are efficient pathways for tracer movement at the well field. The effective porosity for a continuum model between the point of injection and tracer recovery ranges from 2 to 4 percent and is an order of magnitude smaller than previously assumed. Existing well-field protection zones were established using porosity estimates based on specific yield. The effective, or kinematic, porosity of a Biscayne aquifer continuum model is lower than the total porosity, because high velocities occur along preferential flow paths that result in faster times of travel than can be represented with the ground-water flow equation. Tracer tests indicate that the relative ease of contaminant movement to municipal supply wells is much greater than previously considered.

Controls on facies and sequence stratigraphy of an upper Miocene carbonate ramp and platform, Melilla basin, NE Morocco

Abstract Upwelling of cool seawater, paleoceanographic circulation, paleoclimate, local tectonics and relative sea-level change controlled the lithofacies and sequence stratigraphy of a carbonate ramp and overlying platform that are part of a temporally well constrained carbonate complex in the Melilla basin, northeastern Morocco. At Melilla, from oldest to youngest, a third-order depositional sequence within the carbonate complex contains (1) a retrogradational, transgressive, warm temperate-type rhodalgal ramp; (2) an early highstand, progradational, bioclastic platform composed mainly of a temperate-type, bivalve-rich molechfor facies; and (3) late highstand, progradational to downstepping, subtropical/tropical-type chlorozoan fringing Porites reefs. The change from rhodalgal ramp to molechfor platform occurred at 7.0±0.14 Ma near the Tortonian/Messinian boundary. During a late stage in the development of the bioclastic platform a transition from temperate-type molechfor facies to subtropical/tropical-type chlorozoan facies occurred and is bracketed by chron 3An.2n (∼6.3–6.6 Ma). Comparison to a well-dated carbonate complex in southeastern Spain at Cabo de Gata suggests that upwelling of cool seawater influenced production of temperate-type limestone within the ramp and platform at Melilla during postulated late Tortonian–early Messinian subtropical/tropical paleoclimatic conditions in the western Paleo-Mediterranean region. The upwelling of cool seawater across the bioclastic platform at Melilla could be related to the beginning of ‘siphoning’ of deep, cold Atlantic waters into the Paleo-Mediterranean Sea at 7.17 Ma. The facies change within the bioclastic platform from molechfor to chlorozoan facies may be coincident with a reduction of the siphoning of Atlantic waters and the end of upwelling at Melilla during chron 3An.2n. The ramp contains one retrogradational parasequence and the bioclastic platform three progradational parasequences. Minor erosional surfaces that bound the upper surface of the ramp and upper surface of the oldest platform parasequence are related to relative falls in sea level induced by local volcanism and associated tectonic uplift. These local relative falls had little influence on a broader-scale rise to stillstand in relative sea level that controlled development of the transgressive and early highstand systems tracts represented in the ramp and platform, respectively.

New Tertiary stratigraphy for the Florida Keys and southern peninsula of Florida

Seven lithologic formations, ranging in age from Oligocene to Pleistocene, were recently penetrated by core holes in southernmost Florida. From bottom to top, they are the early Oligocene Suwannee Limestone; late-early Oligocene-to-Miocene Arcadia Formation, basal Hawthorn Group; late Miocene Peace River Formation, upper Hawthorn Group; newly proposed late Miocene-to-Pliocene Long Key and Stock Island Formations; and Pleistocene Key Largo and Miami Limestones. The rocks of the Suwannee Limestone form a third-order sequence. Although the entire thickness was not penetrated, 96 m of Suwannee core from one well contains at least 50 vertically stacked, exposure-capped limestone cycles, presumably related to rapid eustatic fluctuations while experiencing tropical to subtropical conditions. The Arcadia Formation is a composite sequence containing four high-frequency sequences composed of multiple vertically stacked carbonate cycles. Most cycles do not show evidence of subaerial exposure and were deposited under more temperate conditions, relative to the Suwannee Limestone. The Arcadia Formation in southernmost Florida is bounded by regional unconformities representing third-order sequence boundaries. Post-Arcadia transgression produced a major backstepping of sediment accumulation above the upper sequence boundary of the Arcadia Formation. The Peace River Formation, composed of diatomaceous mudstones, has been identified only beneath the Florida peninsula and is not present beneath the Florida Keys. Deposition occurred during marine transgressive to high-stand conditions and a local phosphatization event (recorded in northeast Florida). The transgression is possibly related to a global rise in sea level, which resulted in upwelling of relatively cooler, relatively nutrient-rich water masses onto the Florida Platform. It is proposed that the absence of Peace River sediments beneath the Keys is due to sediment bypass of the upper surface of the Arcadia, a result of sediment sweeping by an ancestral Florida current. During late Miocene to Pliocene time in the Florida Keys, siliciclastics of the Long Key Formation and fine-grained carbonates of the Stock Island Formation prograded toward the southern edge of the Florida Platform and downlapped onto the regional unconformity at the top of the Arcadia. Shallow-marine Pleistocene limestones (Key Largo and Miami Limestones), deposited during tropical to subtropical conditions, drape over accretionary successions of the Long Key and Stock Island Formations.

Magnetostratigraphic dating of an Upper Miocene shallow-marine and continental sedimentary succession in northeastern Morocco

Abstract A high-resolution magnetostratigraphy has been developed for an Upper Miocene shallow-marine carbonate complex, equivalent basinal marls, diatomites, and lime mudstones, and an overlying succession of marine to continental carbonate and siliciclastic sedimentary rocks in the Melilla Basin, northeastern Morocco. Units of the carbonate complex, from bottom to top, contain a bryozoan/red algal ramp, bioclastic platform, fringing-reef complex and a unit referred to as the Terminal Carbonate Complex (TCC) [1]. Lithologies from these units contain at least three components of magnetic remanence: (1) a low unblocking temperature component that is likely carried by a viscous remanent magnetization, (2) a low-coercivity, intermediate unblocking temperature component that is probably carried by magnetite or magnetite and maghemite, and (3) a high unblocking temperature component that is probably carried by hematite. Lithologies are typically dominated by the low-coercivity component and single-domain to pseudo-single-domain grain behavior. Rock-magnetic studies suggest biogenic magnetite may contribute to the magnetic remanence of the low-coercivity component. The Melilla Basin magnetostratigraphy, independently corroborated with new 40 Ar 39 Ar dating of volcanic ashes and foraminifer biostratigraphy, has been correlated to the recently developed geomagnetic polarity time scale of Shackleton et al. [2]. This correlation suggests deposition of the bioclastic platform began approximately at the Tortonian/Messinian boundary, which is defined as the base of chron 3Ar [3], and continued into chron 3An.2n. The fringing-reefs span an interval that begins in chron 3An.2n until just above the base of chron 3r (lower Gilbert). The overlying TCC and mixed carbonate and siliciclastic succession correlate to within chron 3r. The new magnetostratigraphy is not without minor ambiguities, possibly due to either remagnetization within the TCC or the presence of a short-duration normal subchron in the lower portion of chron 3r. Results of this study have important implications for correlating between Late Miocene shallow-marine carbonate sections and basinal sections within the Mediterranean Basin, for improving the understanding of the history of the Messinian Salinity Crisis, and for deciphering the roles of eustacy, tectonics, and regional processes in the development of shallow-marine carbonate sequences in the Mediterranean region.

Pathogen and chemical transport in the karst limestone of the Biscayne aquifer: 1. Revised conceptualization of groundwater flow

[1] The Biscayne aquifer is a highly transmissive karst limestone that serves as the sole source of drinking water to over two million residents in south Florida. The aquifer is characterized by eogenetic karst, where the most transmissive void space can be an interconnected, touching-vug, biogenically influenced porosity of biogenic origin. Public supply wells in the aquifer are in close proximity to lakes established by surface mining. The mining of the limestone has occurred to the same depths as the production wells, which has raised concerns about pathogen and chemical transport from these surface water bodies. Hydraulic and forced gradient tracer tests were conducted to augment geologic and geophysical studies and to develop a hydrogeologic conceptual model of groundwater flow and chemical transport in the Biscayne aquifer. Geologic and geophysical data indicate multiple, areally extensive subhorizontal preferential flow zones of vuggy limestone separated by rock with a matrix pore system. The hydraulic response from an aquifer test suggests that the Biscayne aquifer behaves as a dual-porosity medium; however, the results of the tracer test showed rapid transport similar to other types of karst. The tracer test and concurrent temperature logging revealed that only one of the touching-vug flow zones dominates transport near the production wells. On the basis of the rising limb of the breakthrough curve, the dispersivity is estimated to be less than 3% of the tracer travel distance, which suggests that the fastest flow paths in the formation are likely to yield limited dilution of chemical constituents.

A Neogene Mixed Siliciclastic and Carbonate Foundation for the Quaternary Carbonate Shelf, Florida Keys

ABSTRACT New core holes and marine seismic profiles show the distribution and paleoenvironments of a siliciclastic foundation sandwiched between the much-studied Quaternary carbonates of the Middle and Upper Florida Keys and Oligocene to Miocene carbonates of the Arcadia Formation, a distance of some 160 km. Throughout this area the Quaternary carbonates are generally no more than 30 m thick and are underlain by up to 145 m of Plio-Pleistocene mixed limestones and sands, and marine siliciclastics (Peace River Formation). Quartz sands, days, and minor reworked phosphatic grains of the Peace River are the distal deposits of an almost peninsula-wide blanket that interrupted prolonged Tertiary carbonate deposition in southernmost Florida. In the Lower Keys the Peace River siliciclastics change fac es to very fine- to fine-grained carbonates composed of bioclastic, planktonic-foraminiferal grainstones and packstones. The position of the present southern Florida shelf margin, with its discontinuous coral reefs, is determined by the seaward paleoslope of siliciclastics and/or carbonates of the Plio-Pleistocene section defined in the Florida Keys. Lithostratigraphy of five continuous cores on the Keys and southeastern mainland, each about 300 m long, combined with numerous well logs and sample descriptions from the Keys and mainland, establish the southern limit of Peace River siliciclastics to at least the Middle Keys. The well data show intervals of quartz of coarse sand to granule size localized as a north-south pathway of maximum paleocurrents that extend from the central part of the peninsula approximately 350 km to the Middle Keys. This pathway of maximum paleocurrents, about 100 km wide, is interpreted as the palimpsest record of unusually strong, southward-moving shoreline and channeled deposition or a giant prograding spit. The ultimate source of these siliciclastics is from the Appalachians, approximately 1000 km to t e north. We propose that the Plio-Pleistocene marine siliciclastics and carbonates (including the Peace River Formation) recovered in wells on the Keys extend seaward beneath the present southern Florida outer shelf and are present in seismic profiles acquired across the shelf-margin slope. A series of profiles forming an offshore traverse from the Lower to Upper Keys show seaward-dipping clinothems that locally dip obliquely to the arcuate strike of the Florida Keys. These clinothems downlap on a prominent reflector that can be traced to an exposed submarine surface of upper Tertiary carbonates, the Pourtales and Miami Terraces. Some reflectors show configurations similar to contourites, and others, to channels. We contend that southward entry of the siliciclastics into southernmost Florida w s deflected eastward by west-to-east channeled flow of an ancestral Florida Current.

Interplay of Late Cenozoic Siliciclastic Supply and Carbonate Response on the Southeast Florida Platform

ABSTRACT High-resolution seismic-reflection data collected along the length of the Caloosahatchee River in southwestern Florida have been correlated to nannofossil biostratigraphy and strontium-isotope chemostratigraphy at six continuously cored boreholes. These data are interpreted to show a major Late Miocene(?) to Early Pliocene fluvial-deltaic depositional system that prograded southward across the carbonate Florida Platform, interrupting nearly continuous carbonate deposition since early in the Cretaceous. Connection of the platform top to a continental source of siliciclastics and significant paleotopography combined to focus accumulation of an immense supply of siliciclastics on the southeastern part of the Florida Platform. The remarkably thick (> 100 m), sand-rich depositional system, which is characterized by clinoformal progradation, filled in deep accommodation, while antecedent paleotopography directed deltaic progradation southward within the middle of the present-day Florida Peninsula. The deltaic depositional system may have prograded about 200 km southward to the middle and upper Florida Keys, where Late Miocene to Pliocene siliciclastics form the foundation of the Quaternary carbonate shelf and shelf margin of the Florida Keys. These far-traveled siliciclastic deposits filled accommodation on the southeastern part of the Florida Platform so that paleobathymetry was sufficiently shallow to allow Quaternary recovery of carbonate sedimentation in the area of southern peninsular Florida and the Florida Keys.