The Belize margin revisited. 2. Origin of Holocene antecedent topography

The importance of antecedent topography in dictating Holocene facies patterns has been generally recognized. There is, however, disagreement as to origin or lithology of the antecedent topography, particularly with respect to the siliciclastic or carbonate nature of the underlying topography and structural patterns. To help resolve these problems, published and unpublished information have been compiled to produce a structural fabric map of onshore and offshore Belize that includes a new geologic map of the country. The map, along with illustrated seismic lines, demonstrates the occurrence of a number of NNE-trending transpressional faults in which landward directed thrusting is consistently displayed along with tectonic inversion. Offshore wells in conjunction with the seismic lines document the inversion as post-Eocene, suggesting a similar age for transpressional fault movement. Presumably, the landward-directed thrusting reflects the opposing force of eastward-directed subduction along the western margin of Central America relative to the westward seafloor spreading of the Caribbean Cayman Ridge. The Belize faults show little current seismicity, but, nevertheless, the resulting structures have affected Quaternary carbonate deposition as evidenced on an illustrated seismic line by both seaward and landward (bi-directional) progradation of the reef margin from an underlying structural high. The structural influence on the development of Holocene antecedent topography is further suggested by the occurrence of a Bouguer gravity plateau in the same shelf area that marks the occurrence of the Belize lagoon rhomboid shelf atolls. The youngest documented lithology of 12 illustrated offshore exploration wells is Miocene carbonate. In some wells, the carbonate is interpreted as extending into the Pliocene and Holocene although no age diagnostic criteria are in evidence. In other wells, siliciclastics of unknown age and thickness are identified as overlying Neogene carbonates. The regional distribution and age of onshore limestones suggests that unroofing of the Cretaceous carbonate cap of the Maya Mountains siliciclastic sediment source did not occur until late in Neogene time, perhaps no earlier than late Pliocene. Consequently, the Maya Mountains could not have been a major offshore source of siliciclastics until the Quaternary. Information on the lithology of the immediately underlying pre-Holocene is provided by limited penetration core data and shallow resolution seismic lines. These show that antecedent Pleistocene limestones beneath the Holocene reefs were deposited around 130,000 b.p. (isotope stage 5e). The thickness of the overlying Holocene, shelf margin, reef-capped carbonates increases along depositional strike from a few meters in the north to more than 25 m in the south. In contrast, piston cores and seismic data from the southern shelf lagoon collectively document the occurrence of antecedent siliciclastic topography. The southward dip of both carbonate and siliciclastic antecedent surfaces is presumably a reflection of increasing southerly subsidence. Increasing antecedent erosional relief in the same direction reflects the pronounced southerly increase in paleo-rainfall that presumably paralleled that of present-day rainfall.     

DNAPL to LNAPL Transitions During Horizontal Cosolvent Flooding

Cosolvent flooding is a technology with the potential to remove nonaqueous phase liquid (NAPL) sources from the subsurface. It can be used to initiate separate phase mobilization, which allows removal of NAPL within very few pore volumes. Mobilization may result in a sinking DNAPL bank during horizontal flooding of NAPLs denser than water. Reversal of phase density difference between aqueous and DNAPL phases could potentially avoid this downward migration of mobilized DNAPLs. We achieved phase density difference reversal and made DNAPLs float using two components in the cosolvent flooding solution. A low-density cosolvent partitions preferentially into the DNAPL and swells it, which causes a reduction in density of the DNAPL and reversal of the density difference between the NAPL and aqueous phases. A highdensity additive that remains in the aqueous phase allows the cosolvent flooding solution overall to have a density greater than that of water and permits control of the flooding instability. This study focused on tert-butanol as the swelling cosolvent and tetrachloroethylene as the contaminant. In batch tests with sucrose and glycerol as dense additives, phase density difference reversal occurred. To investigate the applicability of phase density difference reversal as a remediation technology, horizontal column and sandbox experiments were performed. These experiments demonstrated the occurrence of phase density difference reversal and effective remediation in horizontal cosolvent floods.     

Holocene cemented beach deposits in Belize

Two types of cemented beach deposits occur on reef islands off the coast of Belize. These are (1) intertidal beachrock that is dominantly cemented by marine aragonite and high-magnesium-calcite cements, and (2) supratidal cayrock that is cemented mainly by vadose low-magnesium-calcite cements. Besides differences in position relative to present sea level and resulting early diagenesic features, beachrock and cayrock can be distinguished on the basis of differences in composition, texture, geographical position, and age. Whereas the composition of beachrock is similar to that of the adjacent marginal reef sediments, cayrock is enriched in benthic foraminifera. Intertidal beachrock is moderately to well sorted and well cemented, while supratidal cayrock is very well sorted, poorly cemented and friable. Beachrock occurs preferentially on windward beaches of sand-shingle Gays on the middle and southern barrier reefs and on the isolated platforms Glovers and Lighthouse Reefs. Cayrock only occurs on larger mangrove-sand Gays of the isolated platforms Turneffe Islands, Lighthouse Reef, and the northern barrier reef. ¹⁴C-dating of ten whole-rock and mollusk shell samples produced calibrated dates between AD 345 and AD 1435 for beachrock and between BC 1085 and AD 1190 for cayrock.

The large-scale distribution of beachrock in Belize supports the contention that physical processes such as water agitation rather than biological processes control beachrock formation and distribution. Only on windward sides of cays that are close to the reef crest, where large amounts of seawater flush the beaches, considerable amounts of cements can be precipitated to produce beachrock. Cayrock forms due to cementation in the vadose zone and is only preserved on larger, stable mangrove-sand cays.     

Accretion patterns in Holocene tropical coral reefs: do massive coral reefs in deeper water with slowly growing corals accrete faster than shallower branched coral reefs with rapidly growing corals?

It is a widely held concept that tropical coral reefs in shallower water with branched acroporid corals should accrete faster than those in deeper water dominated by massive corals. Results from a study of Holocene development of the largest Atlantic reef system, including paleo-waterdepth data, challenge these concepts. In Belize barrier and atoll reefs, reef accretion-rates range from 0.46 to 7.50 m/kyr, and average 3.03 m/kyr, as measured along 33 dated reef sections. Interestingly, accretion-rates increase with increasing paleo-waterdepth, and sections dominated by massive corals accumulated even slightly faster than those with branched acroporids. Published data from some other reef locations reveal no significant trends when plotting reef accretion-rate versus paleo-waterdepth, also indicating that the above-mentioned concepts should be questioned. Massive corals apparently are more resistant and accrete in lower disturbance conditions in slightly deeper water (5–10 m) and higher accomodation (space available for sediment deposition) as compared to shallow water (0–5 m) branched acroporids, which repeatedly get broken and leveled out during tropical cyclones.     

An estimate of water mass transformation in the southern Weddell Sea

Bottom water formation changes the characteristics of water masses entering the southern part of the Weddell Sea through atmosphere-ice-ocean interaction in which both sea and shelf ice play an important role. Modified water, in particular Weddell Sea Bottom Water, recirculates in the west. By comparing the in- and outflowing water masses we have estimated transformation rates on the basis of a data set obtained during the Winter Weddell Gyre Study from September to October 1989. This consisted of a salinity-temperature-depth (CTD) section carried out by R/V “Polarstern” from the northern tip of the Antarctic Peninsula to Kapp Norvegia and data from three current meter moorings maintained from 1989 to 1990 in the eastern boundary current off Kapp Norvegia. Because of the lack of sufficient direct current measurements in the interior and the western boundary current, it was necessary to derive mass transports on the basis of available data combined with physical and geometrical arguments. At the mooring site barotropic currents were measured. They were extrapolated to the interior under the assumption that wind-driven, baroclinic and barotropic current fields are of similar shape. The location of the gyre centre was determined from drifting buoy tracks and geopoten-tial anomaly. A linear current profile from the eastern boundary current to the centre of the gyre was assumed, and the western outflow was determined according to mass conservation. Different assumptions on the transition from the boundary current to the interior and the location of the centre result in a wide range of transports with most likely values between 20 and 56 Sv. The total mass transport was split into individual water masses. Differences between inflow and outflow result in a transformation rate of 3–4 Sv from Winter and Warm Deep Water to Antarctic and Weddell Sea Bottom Water. The net heat and salt transport across the transect implies heat fluxes from the ocean to the atmosphere of 3–10 W m⁻² and ice formation rates of 0.2–0.35 m year⁻¹.     

A nocturnal low level jet during PUKK

A nocturnal low-level jet characterized by a distinct inertial oscillation lasting from around sunset until sunrise the next day was observed during the coastal experiment PUKK. The phenomenon appeared over an area of at least 40 km in diameter; it thus had a mesoscale extension. We were able not only to observe this fine case but also to simulate it by an integrated dynamical PBL model.     

Geochemical controls on a calcite precipitating spring

A small spring fed stream was found to precipitate calcite by mainly inorganic processes and in a nonuniform manner. The spring water originated by rainwater falling in a 0.8 km² basin, infiltrating, and dissolving calcite and dolomite followed by dissolution of gypsum or anhydrite. The Ca²⁺/Mg²⁺ indicates that calcite is probably precipitated in the subsurface from a supersaturated solution. This water emerges from the spring still about 5 times supersaturated with respect to calcite and continues calcite precipitation. When 10 times supersaturation is reached, due to CO₂ degassing the precipitation is more rapid. The calcite accumulation from the stream with a flow of 5 l/s is calculated to be 12600 kg/yr with the highest rates in areas where CO₂ degassing is the greatest. The non-equilibrium, as shown by the high calcite supersaturation, is also reflected in a variable partitioning pattern for Sr²⁺ between the water and calcite.     

Chloritoid-bearing metapelites associated with glaucophane rocks in western Crete,Greece

In western Crete, Greece, a widespread occurrence of chloritoid-bearing metapelites with the main mineral assemblage chloritoid-phengitic white mica-Fe-rich chlorite-quartz was recorded to form the country rock of glaucophane-bearing metabasalts. Six bulk rock analyses of the metapelites conform to the compositional restrictions evaluated by Hoschek (1967) for the formation of chloritoid. Three microprobe analyses revealed chloritoid compositions low in Mg and Mn, and, consequently, high in Fe. The metamorphic grade documented in the metapelites is obviously related to a subsequent prograde metamorphism by which, in the adjacent meta-basalts, epidote is formed at the expense of lawsonite. No relict of a high-P, low-T assemblage, in part well preserved in the meta-basalts, was recognized in the chloritoid schists. The significance of the metamorphic history is briefly discussed.     

Ein verbessertes Strukturmodell für die Ostalpen,abgeleitet aus refraktions-seismischen Daten unter Berücksichtigung des Alpen-Längsprofils

Zusammenfassung Die Vermessung des Alpen-Längsprofils 1975 lieferte unter anderem Daten zur verbesserten Bestimmung der Geschwindigkeits-Tiefen-Verteilung unter dem Alpen-Hauptkamm. Diese Geschwindigkeits-Tiefen-Verteilung ist charakterisiert durch eine mächtige Inversionszone im Tiefenbereich zwischen 20 und 30 km — sie liegt somit tiefer als bei früheren Modellen — sowie eine weitere weniger ausgeprägte Inversionszone in der Unterkruste. Eine mit diesen Ergebnissen konsistente Reinterpretation des EschenloheSüdost-Profils ermöglichte die Konstruktion eines zweidimensionalen Krustenmodells längs der Linie Eschenlohe-Triest. Dieses Modell ist auch in guter Übereinstimmung mit Ergebnissen der Refraktionsprofile Eschenlohe-Ost und Lago Lagorai-Ost. Darüber hinaus befriedigt es Laufzeitbeobachtungen von Nachbeben aus der Region Friaul.

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Refraction seismic measurements along the Alpine Longitudinal Profile 1975 yielded among other things data for a reliable determination of the velocity-depth distribution beneath the crest of the Alps. This velocity-depth distribution is characterized by a rather thick inversion zone in the depth range between 20 and 30 km — which is deeper than with earlier models — as well as another less pronounced one in the lower crust. A reinterpretation of the Eschenlohe-Southeast profile consistent with these new data allowed the development of a two-dimensional crustal model along the line Eschenlohe-Trieste. This model is in excellent agreement with results from refraction profiles Eschenlohe-East and Lago Lagorai-East. It furthermore satisfies travel time observations from aftershocks of the Friulian earthquake.

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Résumé La campagne ALP 1975 a permis entre autres de donner une meilleure détermination de la répartition de la vitesse en profondeur v(z) sous la crête principale des Alpes. Celle-ci est caractérisée par une épaisse zone d'inversion de vitesse (LVZ) entre 20 et 30 km, donc plus profonde que les modèles antérieurs, et une seconde zone d'inversion moins prononcée dans la croûte inférieure. Tenant compte de ces résultats nous avons fait une réinterprétation du profil Eschenlohe-Sud-Est, ce qui nous permet de construire une coupe structurale de la croûte le long de la ligne Eschenlohe-Trieste. Ce modèle est en bon accord avec les résultats des profils Eschenlohe-Est et Lago Lagorai-Est et avec des durées de propagation déterminées pour des répliques du séïsme du Frioul.

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A Lithospheric Seismic Profile Along the Axis of the Alps; paper No. II.