Ivan Gill

Evaporitic mixed-water dolomitization on St. Croix, U.S.V.I.

ABSTRACT Dolomite exists in only a small area of Pliocene carbonates on St. Croix. This area consists of reef, lagoonal, and platform facies that underlie and rim the shoreline of Krause Lagoon, an embayment that existed on the southern coastline of St. Croix prior to industrial development in the 1960s. Since dolomite is not found in similar facies elsewhere on St. Croix, this geographic distribution suggests that the formation of the dolomite was related to hydrologic conditions in Krause Lagoon. The dolomite is calcium-rich (57-60 mol % Ca) and exists both as euhedral rhombs 2-75 µm in diameter and as a replacement mineral in dolomitized bioclasts. Bioclasts in the dolomitic strata often show good preservation of microstructure, particularly in large benthic forams and coralline algae The mean 87Sr/86Sr isotopic composition of the dolomite is 0.70887 ±0.00002 (2s, n = 3) which corresponds to the 87Sr/86Sr ratio of Late Miocene seawater. However, because the dolomite resides in Pliocene strata, it is difficult to invoke unmodified seawater as an agent of dolomitization. Dolomitization therefore requires a source of nonradiogenic strontium. Modern St. Croix groundwater has 87Sr/86Sr compositions between 0.7076 and 0.7085 (n = 4), well below the ratio of both modern seawater and the dolomites. Mixing calculations show that modern St. Croix groundwater could be a significant source of nonradiogenic strontium in a dolomite formed f om a two-component groundwater-seawater mix. On the basis of strontium-concentration modeling, the groundwater component responsible for the St. Croix dolomites may have ranged from 40% to 80% of the dolomitizing fluids. Stable isotopic values for the dolomite range from d18O of +0.7 to +3.8, and d13C of +0.6 to +2.4 (PDB), with increasing d18O and d13C values from the margins to the center of the lagoon. The maximum d18O values in these dolomites are too high to have formed fro unaltered groundwater or seawater, even accounting for ice-volume effects. Therefore the isotopically heaviest dolomite must have precipitated from fluids enriched in 18O, probably as a result of evaporation. Dolomitization from fluids produced from a mixture of evaporated seawater and St. Croix groundwater are consistent with the geochemistry and geologic distribution of the dolomite. Calculations show that such a scenario is possible, and may be fairly common, despite the relative complexity of the model. Documented block faulting of the Krause Lagoon area may have provided a stable hydrologic regime for a long enough time for dolomite to form, despite island uplift during the late Tertiary. Other models of dolomitization can be shown to be less likely or untenable on the basis of chemical, lithologic, or hydrologic criteria.

The Role of Framework in Modern Reefs and Its Application to Ancient Systems

Uniformitarianism — the principle that “the present is the key to the past” — forms the cornerstone of geologists’ efforts to unravel the prehistory of our world. When comparing modern and fossil reefs, a fundamental problem arises because the organisms that largely drive this important biological system have evolved over geological time. This has fueled a heated debate over the appropriateness of using modern reefs dominated by scleractinian corals as models for ancient ones built by stromatoporoids, sponges and a host of seemingly dissimilar organisms.

Holocene reef building on eastern St. Croix, US Virgin Islands: Lang Bank revisited

New core and seismic data suggest that widespread reef building started on Lang Bank by 8,900 CalBP and was dominated by Acropora palmata for the next three millennia. Accretion rates averaged 5.81xa0m ky−1, a rate that was sufficient for reefs to keep pace with rising sea level on the bank throughout their history. Seismic data show a deep platform interior that was flooded well in advance of reef building along the elevated rim. As a result, those reefs were buffered from sediment stress by their higher positions and active water flow to the west. A. palmata disappeared from the shallow margin by 6,350xa0yr ago, and reef building on Lang Bank largely ceased by 5,035 CalBP. The reasons for these dramatic events are unclear. Water depth over the reefs was generally shallower than when they started to build, and sea level was slowing dramatically. The new data described here show that reefs flourished on Lang Bank throughout the hiatus suggested by earlier studies (10–7 kyrs BP), and the ultimate demise of shelf-edge reefs is clearly not associated with either poor water quality or sudden sea-level rise. In addition, accretion rates from eastern St. Croix and throughout the Caribbean were well below the high values (≥10xa0m ky−1) that have been widely assumed. These data collectively argue against models that require extreme environmental or oceanographic phenomena to drown reefs on Lang Bank where reef building was too fast to be outpaced by Holocene sea-level rise. This also bears on more generalized Caribbean models that depend on the presumed reef history on eastern St. Croix.

Why here and why now? Teacher Motivations for Unionizing in a New Orleans Charter School"

The rigidity of teachers unions has been given as a primary reason for their lack of representation among America’s rapidly growing, although still relatively small, charter school sector. In the case of post-Hurricane Katrina New Orleans, the city rapidly converted from a union-backed teacher workforce to a largely nonunionized charter school workforce in the years following state takeover and charter conversion. This makes the recent emergence of two single-school unions in charter schools there worthy of study. As the teachers attempt to organize single-school unions in a nearly all charter school system, what are their motivations? This case study of one of New Orleans’ emerging charter school unions found that pay inequities, job insecurity, a lack of teacher voice in school-level decisions, and a culture of compliance, all motivated teachers to seek unionization. Teachers hoped to promote equity and teacher involvement with their union, but the organizing effort did strain some relationships, particularly those involving middle management.

Development of Miocene-Pliocene Reef Trend, St. Croix, U.S. Virgin Islands: ABSTRACT

The Miocene-Pliocene reef trend on St. Croix, U.S. Virgin Islands, rims the present southern western coasts of the island and includes accompanying lagoonal and forereef facies. The reef trend was established on a foram-algal bank facies that represents basinal shallowing from the deep-water pelagic and hemipelagic facies of the Miocene Kingshill Limestone. Information on facies distribution and thickness is derived from rock exposures and 22 test wells drilled to a maximum depth of 91 m. The greatest thickness of the reef facies exists in a subsidiary graben on the south coast of St. Croix. The thickness of the reef section in this locality is due to preservation of the section in a downdropped block. Reef faunas include extant corals, as well as several extinct genera. Extant corals (e.g. Montastrea annularis, Diploria sp., and Porites porites) and extinct corals (e.g., Stylophora affinis, Antillea bilobata, and Thysanus sp.) are the main reef frame-builders. Coralline algea and large benthic foraminifera are significant contributors to the sediments both prior to and during scleractinian reef growth. Dolomitization and calcite cementation occur prominantly in an area corresponding to a Holocene lagoon. The spatial distribution of the dolomite suggests that the lagoon is a Tertiary feature directlymorexa0» related to the dolomitization process. Stable isotopic values suggest dolomitization of fluids of elevated salinity.«xa0less

Accretionary Styles in Shelf-Edge Reefs, St. Croix, Virgin Islands: ABSTRACT

Fourteen reef cores were taken in water depths of 8-30 m (25-100 ft) along two contrasting shelf margins on St. Croix, Virgin Islands. Seven of the cores were drilled vertically into the deep reef rimming the narrow (150-300 m, 500-1,000 ft) shelf of Cane Bay. Seven horizontal cores were drilled into the steeply sloping reef walls of Salt River submarine canyon. The Cane Bay cores indicate initial Acropora colonization on a rubble slope of 5,000-6,000 years B.P. (at a water depth of 5 m or 16 ft). A subsequent 3,000-year hiatus in reef accretion was ended by recolonization of head corals (at a water depth of 8 m or 25 ft). The Salt River cores show alternating Montastrea annularis framework and open or sediment-filled voids up to 2 m (6 ft) across. This pattern is related to the complex system of channels, caverns, and overhangs common in this steep reef face. At Cane Bay, vertical accretion rates averaged 1.16 m/1,000 years (45.7 in./1,000 years), with a range of 0.17-1.69 m/1,000 years (6.7-66.5 in./1,000 years). Lateral accretion rates at Salt River averaged 1.03 m/1,000 years (40.6 in./1,000 years), with a range of 0.84-1.38 m/1,000 years (33.1-54.3 in./1,000 years). The surprisingly high lateral accretion rates at Salt River are largely the result of down-faulted reef blocks causing repetitions of section. Comparison of reef-accretion rates based on the cores with calcification potential for the reef (primarily as coralgal growth) indicates that more than 60% of the carbonate incorporated in the reef is ultimately reduced to sediment. Unlike that on Pacific reefs, this material is moved seaward and off the shelf edge, primarily during major storms. Such early degradation of the reef relates to the under representation of reef framework in the ancient record. End_of_Article - Last_Page 488------------