Paul Enos

Timing of recovery from the end-Permian extinction: Geochronologic and biostratigraphic constraints from south China

Four volcanic-ash beds bracket the Early-Middle Triassic boundary, as defined by conodont biostratigraphy, in a stratigraphic section in south China. High-precision U-Pb dates of single zircons allow us to place the Early to Middle Triassic (Olenekian-Anisian) boundary at 247.2 Ma. Magnetic-reversal stratigraphy allows global correlation. The new dates constrain the Early Triassic interval characterized by delayed biotic recovery and carbon-cycle instability to ∼5 m.y. This time constraint must be considered in any model for the end-Permian extinction and subsequent recovery.

Hurricane Betsy in the Florida-Bahama Area: Geologic Effects and Comparison with Hurricane Donna

Within a 5-year period two violent tropical storms, Hurricanes Donna and Betsy, passed over the Florida Keys. Although these hurricanes were of comparable size and intensity, their geologic effects differed. Both hurricanes caused extensive damage to the outer reefs, although Betsy acted essentially on a fauna from which Donna had removed the weaker elements. Large-scale movement of sand on White Bank which resulted from Hurricane Donna did not occur during Betsy. In Florida Bay, supratidal sedimentation on the islands and mainland was extensive during Donna, but virtually absent during Betsy. An exception to this was the deposition of spillover lobes of skeletal sand landward of the beach on Cape Sable. Unlike Donna, Hurricane Betsy passed over the northwest portion of the Great Bahama Bank. Large spillover lobes of oölitic sand were eroded back as much as 140 feet. Sedimentation on the Andros tidal flats was restricted to vegetated highs, even though the entire tidal flat complex was inundated by as much as 9 feet of water.

Evolution of Florida Bay from island stratigraphy

The sedimentary record of most Florida Bay islands is an asymmetric cycle consisting of a transgressive sequence followed by a regressive sequence, both formed during a continuous Holocene rise in sea level. The principal sedimentary environments of Florida Bay and the south Florida mainland are represented in the cycle by an upward succession of (1) freshwater pond, (2) coastal mangrove swamp, (3) shallow bay (“lake”), (4) mud bank, and (5) island. Some parts of the cycle may be missing, but the sequence is always the same. Supratidal carbonate sedimentation on islands may develop from coastal mangrove swamps or by mangrove colonization of mud banks. Islands have developed from mud banks at many different times during the rise of sea level into Florida Bay, indicating that mud banks must have existed throughout most of the history of the bay. Present trends of island formation and growth suggest that Florida Bay will evolve into a coastal carbonate plain with inland mangrove swamps and freshwater ponds, very similar to the present southwest Florida mainland.

Anatomy of a Flysch

ABSTRACT Detailed study of a flysch sequence provides information on the variability and spatial relations of characteristic sedimentary structures, on the geometry of beds, and on the dynamics of the depositing currents. Excellent exposures of the Middle Ordovician Cloridorme Formation in northern Gaspe Peninsula permit correlation of individual beds up to 7.5 km and well defined units up to 10.5 km in the direction of current flow. Flutes and grooves are the most abundant directional structures. The orientation of grooves is more variable than that of flutes in vertical section (S.D. of ± 30-37° vs ± 19°) and laterally within individual beds (± 28-31° vs ± 11-18°). The variability of direction indicators is greater in graywackes than in either finer-grained or more calcareous rocks and is greater in vertical section than laterally within individual beds. Few longitudinal trends in the development of sedimentary structures were detected, but locally eroded fragments of the finer-grained sediments are important as textural elements, as tools, and as erosional indicators. Individual graywacke and calcisiltite beds are very discontinuous in longitudinal section. Beds disappear downcurrent at a rate of about 50 percent in 4 km; the number of beds within a unit decreases by 50 percent in 10 km. The thickness of the small units decreases about 7 percent per km. Local current bypassing, imbrication of boils, and differential current deposition of argillite produce an intricate mosaic in longitudinal section. A volume of 10 to 30 106 m3 is estimated for an average graywacke bed.

Death in Guizhou — Late Triassic drowning of the Yangtze carbonate platform

Abstract The Yangtze platform in south China formed a stable palaeogeographic element from the Late Proterozoic to the end of the Middle Triassic with deposition of shallow-water carbonates during much of this time. A portion of the Yangtze platform in south-central Guizhou drowned at the transition from Permian to Triassic, as the south-adjacent Nanpanjiang basin encroached about 100 km northward, but a new, stable platform margin was established that persisted through the Early and Middle Triassic. This long history as a stable carbonate platform ended at the transition from the Ladinian to the Carnian. The latest Ladinian rocks, the Yangliujing Formation, are 490 m of shoaling-upward carbonate cycles of grapestone and bioclastic grainstone, fenestral limestone, and stromatolitic dolomudstone, commonly overprinted by extensive subaerial diagenesis. The beginning of the Carnian is marked by a rapid transition to medium-dark-grey, nodular lime mudstones containing ammonoids, conodonts and thin-shelled bivalves, the Zhuganpo Formation. The upper part of this thin pelagic limestone contains many muddy intraclasts, some slightly bored and encrusted, indicating incipient cementation. The overlying Wayao Formation is a condensed black shale with thin interbeds of dark-grey, manganiferous lime mudstone near the base. Ammonoids, conodonts, thin-shelled bivalves, and articulated crinoid stems are abundant. Fine-grained greywacke with sole marks forms prominent bundles within grey, calcareous shale in the overlying Laishike Formation. Ammonoids and thin-shelled bivalves occur sporadically in this 810-m-thick unit. Calcareous shale with thicker-shelled bivalves and packages of cleaner, coarser-grained sandstone characterize the Banan Formation, 460 m thick. The sandstone units generally coarsen and thicken upward, with ripples, medium-scale trough cross-beds, and rare U-tube burrows. Quartzose, coal-bearing siliciclastics 690 m thick form the overlying Huobachong Formation. Thick-bedded, cross-stratified sandstone and conglomerates are amalgamated into thinning- and fining-upward intervals separated by blocky mudstones. This fining-upward motif continues into the overlying Erqiao Formation, but coals are lacking. At the beginning of the Late Triassic (Carnian) the previously stable Yangtze platform, on which peritidal limestones were forming, was drowned and covered by dark lime mud that was cemented into intraclasts and nodular lime mudstone. Black shale and manganiferous pelagic limestone formed a condensed interval, recording maximum submergence. Turbidite sandstone and shale of the Laishike flysch filled the accommodation space of 800 m created during drowning of the Yangtze platform, leading to deposition of shoaling-upward shelf and paralic sandstones and shales, but without significant carbonate production. The succeeding fining-upward siliciclastics are interpreted as braided-stream deposits with coals that mark minor marine incursions. The shallow-shelf and braided-stream deposits form a molasse 1500 m thick. It was apparently derived from the west, in contrast to the underlying flysch where palaeocurrent directions are from the north or northeast. The entire Yangtze platform became emergent during the Late Triassic and was never submerged again. Subtle local differences in the drowning sequences indicate differential subsidence and suggest that tectonics played a role in the death of the Yangtze platform.

Lofer cyclothems revisited (Late triassic, Northern Alps, Austria)

SummaryFischers classic study (1964) of the Upper Triassic “Lofer” cyclothems in the Dachstein Limestone of the Northern Calcareous Alps was seminal to many studies of the Dachstein and to carbonate cycles globally.Fischers idealized cycle is deepening upward, ABC in his terminology, where member A is a surface or interval of subaerial exposure, B is tidal deposits, and C is shallow subtidal. Studies of the Dachstein from the elsewhere in the northern Alps have substantiatedFischers upward-deepening ABC cycle, butGoldhammer et al. (1990) andSatterley (1996a) reinterpreted the type Lofer cycles as shoaling upward.We measured 139 m of Dachstein Limestone incorporating 25 cycles at Steinernes Meer, Austria, nearFischers most extensive section. In this section we identified no A members. The section is punctuated by slightly reddish horizons (‘pink partings’) that in some cases may reflect brief subaerial exposure, but generally appear to be pressure-solution zones that have concentrated iron oxides but lack a distinctive isotopic signature. B members are readily distinguished by fenestral porosity, stromatolitic lamination, partial dolomitization, intraclasts, or desciccation cracks. They are relatively thin (5 to 155 cm; median thickness is 38 cm.) and in some cases laterally variable or discontinuous. C members are characterized by molluscan wackestones and packstones with diverse biota. C intervals are 25 cm to 26 m thick (median 4.1 m) and comprise 91% of the interval measured. Pervasive bright-red internal sediment, which appears commonly within the B and C members, does not derive from any interval observed within the measured section, but from sources, possibly paleosols, much higher in the section. It is spatially associated with near-vertical, ENE-trending (62o) fractures filled with the sediment, brachiopods, and cement. Such fractures cut stratigraphic intervals as thick as 70 m without an exposed top or base.If “pink partings” were accepted as indicative of subaerial exposure, three cyclothems (12%) would correspond toFischers ideal upward-deepening cyclothem, seven cyclothems (29%) are shoaling-upward, four (17%) are symmetrical and the remaining 10 (42%) are incomplete with both deepening and shoaling components. If subaerial disconformities are absent, the intervals are better described as BCBC rhythms than as true cycles. Our study is intended to stimulate new discussion of patterns and origin of the Lofer cycles.

Shallow-marine phreatomagmatic eruptions through a semi-solidified carbonate platform (ODP Leg 144, Site 878, Early Cretaceous, MIT Guyot, West Pacific)

Abstract Leg 144 (1995) of the Ocean Drilling Program recovered basalts and volcaniclastic material from the volcanic basement of several guyots in the northwest Pacific, including MIT Guyot in site 878. Tectonic reconstruction suggests that this seamount originated as an intraplate volcano on the Pacific Superswell in the Early Cretaceous. Tephra emplaced by mass flow within the sedimentary cap is the subject of this study. The tephra unit consists of palagonitized, moderately to strongly vesicular basaltic clasts of apparent phreatomagmatic origin, carbonate mud- to grainstone clasts, and armoured lapilli. Armoured lapilli are inferred to have been formed during eruption within a steam envelope that developed above the vent due to magma–water interaction and high gas content of the magma. Another distinctive feature of the tephra unit are abundant truncation surfaces that cut the lithotypes due to subsidence, and are juxtaposed to the same lithotypes. These truncation surfaces are inferred to represent intra-eruption slip surfaces, which formed during eruption of the volcano that generated the tephra pile. The resulting slumps initiated grainflows and dilute turbidity currents, whose deposits are interspersed with those of high-concentration density currents. A subaqueous setting of deposition and eruption is indicated by the abundance of sideromelane glass shards, cauliflower-chilled clasts, the underlying marine carbonates, the lack of tachylite within the deposits, and sedimentary structures within the breccia.

Carbonate production of the coral Diploria labyrinthiformis in south Florida patch reefs

Abstract Average annual production of CaCO3 (aragonite) by the hermatypic coral Diploria labyrinthiformis in the Florida reef tract is 11.8 ± 0.3 kg/m2 of reef space occupied by the coral. Annual production was estimated from a population of 271 Diploria colonies measured in situ. The mass produced annually is the volume of annual growth times the skeletal density. Cores of thirty-one Diploria colonies were slabbed and X-rayed to reveal annual growth bands. The mean vertical growth rate is 0.35 ± 0.06 cm/yr for skeletal density bands representing up to 34 years of growth. The annual volume of growth is approximated by a simple geometric shell whose thickness is determined by the horizontal and vertical growth rates of the colony. Horizontal growth rates are estimated from the ratio of the two lateral axes to the vertical axis. Bulk density determined by dry weight/volume of 9 cyclindrical cores is 1.64 ± 0.13 g/cm3. Productivity of 11.8 kg/m2/yr for area occupied by the coral does not vary appreciably among a nearshore patch reef and two outer-shelf patch reefs. For comparison, production rates for 244 Porites astreoides near Key West, Florida, is 14 ± 3.3 kg/m2/yr for area occupied by the coral. The 271 colonies of D. labyrinthiformis potentially produce sediment which, if spread evenly, would accumulate at a rate of 0.026 m/103 yr for 9008 m2 of reef surveyed. Potential vertical accretion by all corals for the patch reef surveyed is estimated at 2.2 to 6.6 m/103 yr.

Lower Cretaceous peritidal limestones at 2,700-m depth, Blake nose, Atlantic Ocean

At sites 390 and 392 (Deep Sea Drilling Project, Leg 44) on the Blake nose, thoroughly lithified Lower Cretaceous limestone more than 250 m thick is abruptly overlain by a condensed sequence of Barremian to Eocene pelagic carbonate ooze. The Lower Cretaceous sediments consist of three units: limestone with moldic porosity (base), oolitic limestone, and fenestral limestone. Subaerial diagenesis of the limestone section is recorded by (1) caverns with vertical dimensions of up to 10 m, (2) stalactitic intergranular cement, and (3) meniscus sediment (or cement). Compatible with these subaerial features are mud cracks, fenestral fabrics, intraclasts, and cryptalgal structures. Inasmuch as these shallow-water and tidal-flat deposits are now beneath 2,607 m of sea water (plus 99 m of younger sediments), they serve to dramatize the apparent degree of Barremian and later subsidence of this part of the Atlantic outer continental shelf. Porosity and permeability are high in vuggy samples, which are common in the skelmoldic limestone. Cementation has destroyed most of the extensive primary porosity of the two younger units.

Sedimentary Structures Preserved in Greater Himalayan Gneiss: Phenomenon or Fantasy?

Under favorable conditions sedimentary structures may be preserved in high-grade metamorphic rocks to yield valuable clues about the depositional and paleogeographic setting of the protolith. Two examples from gneisses of the Greater Himalayan Sequence of Bhutan demonstrate this potential. Laminated, channeled dolostone within gneiss of upper amphibolite facies indicates tidal-flat deposition and thus proximity of a paleoshoreline. The type Takhtsang Gneiss, within granulite facies, consists of alternating millimeter bands rich in quartz and biotite, interpreted as silt and clay interlaminations, respectively, in the protolith, typical of tidal deposits or varves. The gneissic banding drapes around isolated “lenses” of contrasting and varied compositions, interpreted as dropstones. This combination suggests deposition in a varved glacial lake. Published comparisons of Takhtsang lithologies indicate a minimum dimension of 315 km for the postulated lake. These examples demonstrate the potential of relict sedimentary features in reconstructing geologic history, even in high-grade metamorphic terrains.