Giant flute-like scour and other erosional features formed by the 1929 Grand Banks turbidity current

Erosional features on the floor of Eastern Valley of the Laurentian Fan, in 2800 m water depth, have been mapped with SeaMARC I side-scan sonar images and Seabeam multi-beam echo-soundings, and were directly observed during a dive with the deep submersible Alvin. The most spectacular feature is a 100-m-deep flute-shaped scour, more than 1 km long. The surrounding valley is floored by an unconsolidated coarse conglomerate, which was moulded into transverse bedforms by the turbidity current that was triggered by the 1929 Grand Banks earthquake. Direct observations and seismic-reflection profiles show that the flute-shaped scour cuts through this conglomerate and into Plio-Pleistocene valley-floor sediments, thereby exposing a section through the 1929 deposit. Application of the Allen defect theory suggests that the flute is unusually deep because general channel-floor erosion was inhibited by the conglomerate veneer. Valley-floor channels typically 1 km wide and 10m deep contain series of closed depressions that occasionally deepen to 30 m. These are also interpreted as erosional scours, analogous to pools cut on the beds of bedrock rivers. The large flute was probably formed by detached flow enlarging an initial scour depression. Such scours probably play an important role in channel-floor erosion, increasing the volume of sediment transported by large turbidity currents.     

Flume experiments on the alignment of transverse, oblique, and longitudinal dunes in directionally varying flows

For more than a century geologists have wondered why some bedforms are orientated roughly transverse to flow, whereas others are parallel or oblique to flow. This problem of bedform alignment was studied experimentally using subaqueous dunes on a 3–6-m-diameter sand-covered turntable on the floor of a 4-m-wide flume. In each experiment, two flow directions (relative to the bed) were produced by alternating the turntable between two orientations. The turntable was held in each orientation for a short time relative to the reconstitution time of the bedforms; the resulting bedforms were in equilibrium with the time-averaged conditions of the bimodal flows. Dune alignment was studied for five divergence angles (the angle between the two flow directions): 45°, 67–5°, 90°, 112–5° and 135°. The flow depth during all experiments was approximately 30 cm; mean velocity was approximately 50 cm s^-1 and mean grain diameter was 0–6 mm. Each experiment continued for 30–75 min, during which time the flume flow was steady and the turntable position changed every 2 min. At the end of each experiment, water was slowly drained from the flume and dune alignment was measured. Transverse dunes (defined relative to the resultant transport direction) were created when the divergence angle was 45° and 67–5°, and longitudinal dunes were created when the divergence angle was 135°. At intermediate divergence angles, dunes with both orientations were produced, but transverse dunes were dominant at 90°, and longitudinal dunes were dominant at 112–5°. One experiment was conducted with a divergence angle of 135° and with unequal amounts of transport in the two flow directions. This was achieved by changing the orientation of the turntable at unequal time intervals, thereby causing the amount of transport to be unequal in the two directions. The dunes formed during this experiment were oblique to the resultant transport direction. These experimental dunes follow the same rule of alignment as wind ripples studied in previous turntable experiments. In both sets of experiments, the bedforms developed with the orientation having the maximum gross bedform-normal transport (the orientation at which the sum of the bedform-normal components of the two transport vectors reaches its maximum value). In other words, the bedforms develop with an orientation that is as transverse as possible to the two flows. In those cases where the two flows diverge by more than 90° and transport equal amounts of sand, bedforms that are as transverse as possible to the two separate flows will be parallel to the resultant of the two flow vectors. Although such bedforms have been defined by previous work as longitudinal bedforms, they are intrinsically the same kind of bedform as transverse bedforms.     

Anorthosites and Related Megacrystic Units in the Evolution of Archean Crust

Archean anorthositic complexes occur in essentially all Archeancratons and contain large equidimensional plagioclase crystals(up to 30 cm. diam.) with highly calcic compositions (An₈₀ toAn₉₀) but are not readily amenable to determination of theirparent melt compositions. However, insight into petrogenesisof the complexes is provided by megacrysts of plagioclase thatare identical to those in the complexes and occur in many Archeanflows, sills, and dikes whose matrices display REE and fractionationpatterns that indicate tholeiitic trends and are compatiblewith prior subtraction of plagioclase during earlier evolutionof the melts. Included blocks of anorthosite and megacrystswith very thin rims that approach the more sodic compositionof lathy plagioclase in the matrices indicate an earlier stageof cryst formation under different conditions of crystallizationthan the matrices. The megacrystic units occur both in greenstonebelts that have oceanic affinities and stable cratonic dikeswarms that have continental affinities. Both major and traceelement contents of the matrices of the megacrystic units differbetween greenstone and cratonic dike environments; the dikesbeing higher in Si0₂, TiO₂ FeO, Na₂, K₂O, and light REEs butlower in Al₂O₃ and CaO. The matrices of both environments followseparate but parallel tholeiitic fractionation with high Fe-enrichmenttrends similar to Skaergaard liquids suggesting relatively lowvolatiles and fo₂. Experimental data and projections in CMAFspace suggest a multistage petrogenesis involving a relativelyhigh-pressure fractionation of olivine and/or orthopyroxenefrom a primitive mafic melt followed by ascension of the fractionated,less-dense melt, probably in several pulses, to a low-pressurechamber, probably at 1 to 2 kb. The depressurization accompaniedby cooling could easily place the melt composition in the plagioclasefield and significantly below the liquidus resulting in severalcrystallization cycles of plagioclase in the low pressure chamber.The melts would crystallize as anorthositic complexes and periodicallyexpel pulses that would form the observed megacrystic flows,sills, and dikes.     

Lower Cambrian shelf and shelf margin buildups,Flinders Ranges,South Australia1

Carbonate buildups in the Flinders Ranges of mid-Early Cambrian age grew during a period of high archaeocyath diversity and are of two types: (1) low-energy, archaeocyath-sponge-spicule mud mounds, and (2) high-energy, archaeocyath-calcimicrobe (calcified microbial microfossil) bioherms. Mud mounds are composed of red carbonate mudstone and sparse to abundant archaeocyath floatstone, have a fenestral fabric, display distinct stromatactis, contain abundant sponge spicules and form structures up to 150m wide and 80 m thick. Bioherms are either red or dark grey limestone and occur as isolated small structures 2–20 m in size surrounded by cross-bedded calcarenites and calcirudites or as complexes of mounds and carbonate sands several hundreds of metres across. Red bioherms comprise masses of white Epiphyton with scattered archaeocyaths and intervening areas of archaeocyath-rich lime mudstone. Grey bioherms are complex intergrowths of archaeocyaths, encrusting dark grey Renalcis and thick rinds of fibrous calcite cement. The bioherms were prone to synsedimentary fracturing and exhibit large irregular cavities, up to 1.5 m across, lined with fibrous calcite. The buildups are isolated or in contiguous vertical succession. Mud mounds occur alone in low-energy, frequently nodular, limestone facies. Individual bioherms and bioherm complexes occur in high-energy on-shelf and shelf-margin facies. The two types also form large-scale, shallowing-upward sequences composed of basal (deep water) mud mounds grading upward into archaeocyath-calcimicrobe bioherm complexes and bioherms in cross-bedded carbonate sands. The uppermost sequence is capped by ooid grainstone and/ or fenestral to stromatolitic mudstone. The calcimicrobe and metazoan associations form the two major biotic elements which were to dominate reefs throughout much of subsequent Phanerozoic time.     

Patterns of sediment composition of Jamaican fringing reef facies

Recent carbonate sediments from Jamaican north coast fringing reefs were collected along three parallel traverses in the vicinity of Discovery Bay. Each traverse extended from near shore across the back reef, reef crest, and fore reef to a depth of 75 m. Relative abundances of the biotic constituents vary between sites, reflecting general patterns of reef community composition. The sediment is dominated by highly comminuted coral fragments (27·1% to 63·1%), plates of the calcareous green alga Halimeda (0·4% to 38·7%), coralline algae (4·7% to 16·2%) and the encrusting foraminiferan Homotrema rubrum (0·7% to 9·5%), with lesser amounts of other taxonomic groups (non-encrusting foraminifera 1·3–5·5%; molluscs 1·4–7·0%; echinoderms 0·9–5·0%). Coral fragments, coralline algae and particles of Homotrema rubrum dominate the sediments of the shallow portions of the fore reef (5–15 m), whereas plates of Halimeda are most abundant in sediments from the back reef and deeper portions of the fore reef ( 24 m). Q-mode cluster analysis, using sediment constituent data, resulted in the delineation of four reef biofacies over the depth range of this study (1–75 m).     

Ultra-fine structures and genesis of the Campanian Negev high-grade phosphorites (southern Israel)

Ultra-fine structures of the Negev high-grade phosphorites provide valuable clues to unravelling the genesis of these rocks, the question of their differential areal distribution, and the biosedimentary mechanisms involved in their considerable enrichment. Peculiar to these phosphorites is an intergranular phosphate matrix, for the most part constituted by a variety of phosphatic microbial tubules displaying a range of spatial micro-organizations. The phosphate particles fixed by this filamentous meshwork also consists of internally organized and non-organized packed microbial remains of different types. Analogies between the fabrics of the matrix and the corpuscles lead to conception of a two-stage depositional scheme for these phosphorites, based on rhythmical repetition of two sedimentary mini-events, in a slightly oscillating, very shallow marine system—(1) a low-energy event of microbial colonization of the Mishash bottoms, followed by early phosphatization of the organic structures mainly in marginal situations; and (2) a higher-energy event which broke up the phosphatic mats into debris, redepositing them as clastic layers in nearby basinward sites, while becoming bound by a new meshwork of filamentous microphytes. The differential phosphatization of the intergranular microbial binder, again occurring mostly in marginal localities, produced highly enriched phosphorites. Minor truncations and redepositions leading to amalgamation of the successive layers account for the massive fabric now displayed by most of these rocks. Examining the structural and textural features, the validity of the sedimentary mechanisms of Recent phosphorite formation for the Campanian Negev rocks is discussed.     

Petrologic Evolution of Large Failed Rifts in the Eastern Pacific: Petrology of Volcanic and Plutonic Rocks from the Mathematician Ridge Area and the Guadalupe Trough

The Mathematician failed rift was a fast spreading oceanic ridgeprior to abandonment about 6.5 Ma. but presently has the morphologyof a slow spreading ridge crest. Dredges of its inner rift valleywalls indicate that it also has the petrologic characteristicsof a slow-spreading ridge: the dredges recovered serpentinizedperidotite, cumulate and isotropic gabbro, diabase and mid-oceanridge basalt (MORB) lava. The gabbros include fresh troctolites,cpx gabbros and two pyroxene gabbros, all of which have relativelyiron-rich mafic minerals and sodic plagioclase. Other gabbrosamples are sheared and metamorphosed and presently have lowergreenschist to lower amphibolite mineral assemblages. Peak metamorphictemperatures calculated from coexisting amphibole and plagioclaseare about 600 ?C at 1-2 kb pressure but many of the assemblagesreflect much lower temperatures. The MORB lavas from the riftvalley walls are relatively fresh and very primitive (100 ?Mg/(Mg?Fe^2?) = 72–74). This suggests that they were eruptedin the last stages of rift abandonment after the disappearanceof the shallow crustal magma chamber that may have been presentwhen the rift was a fast-spreading one. One dredge of the Guadalupefailed rift also recovered MORB lava, but it is more evolvedthan that at the Mathematicians. Dredges of failed transformfaults in the Mathematicians recovered fresh alkali basalt.This post-abandonment alkali basalt may be analogous to alkaliclavas erupted off-ridge near active spreading ridges like theEast Pacific Rise. Dredges of an E-W bathymetric linear featurenear the Mathematician ridge south of the Clarion fracture zonealso recovered very fresh alkalic lavas. These are distinctfrom the post-abandonment alkalic rocks in the Mathematiciansand include hawaiites and trachytes, some of which were ‘popping’when first brought to the surface. We conclude that in somecases of fast-spreading rift abandonment, the rift first slowsdown and ceases to have a steady state magma chamber in thecrust. This is followed by the eruption of alkali basalts atfailed transforms as the lithosphere thickens in place.     

Erosion and deposition of fine-grained sediments from the Bay of Fundy

Simulations of the erosion, transport and deposition of fine-grained sediment, such as that of Greenberg & Amos and the Hydraulics Research Station, have illustrated a general lack of reliable field data. Consequently, some standard equations and constants used in modelling the sedimentation character of fine-grained cohesive sediment were evaluated based on data from two field studies and a flume experiment with undisturbed sediment from the Bay of Fundy. Initial results showed that the resistance to erosion of intertidal fine-grained sediment is controlled largely by the degree of subaerial exposure and the consequent dehydration and compaction. The sediment shear strength was high (4 kPa), but generally decreased seawards across the intertidal zone. The resistance of intertidal mud to erosion can be 80 times greater than sub-tidal counterparts. The rate of sediment erosion varied as a complex function of the applied bottom shear stress. At stresses immediately above the critical, the erosion rate decreased asymptotically with time. At higher excess stresses, the erosion rate was linear with respect to time. Thus sediment erosion cannot be represented by a single coefficient. The Krone method of computing sedimentation rates of suspended material was shown, by comparisons with direct measurement, to overpredict by 29%. All variables used in his method were measured in the evaluation with the exception of the critical deposition stress (τ_d). The closest comparisons were obtained when τ_d was assigned a value of 0.1 N m^?2 following Creutzberg & Postma. The in situ still-water particle settling rate (V_o) was constant with respect to time (2.1 × 10^?3 m s^?1). However, the settling tube measures of settling rate, compared to in situ results, underpredicted particle settling by an order of magnitude (2.7 × 10^?4 m s^?1). The reason for this discrepancy is not apparent from our results.