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Garson, D.E., Gaines, R.R., Droser, M.L., Liddell, W.D. & Sappenfield, A. 2011: Dynamicpalaeoredox and exceptional preservation in the Cambrian Spence Shale of Utah. Le-thaia, DOI:10.1111⁄j.1502-3931.2011.00266.xBurgess Shale-type faunas provide a unique glimpse into the diversification of metazoanlife during the Cambrian. Although anoxia has long been thought to be a pre-requisitefor this particular type of soft-bodied preservation, the palaeoenvironmental conditionsthat regulated extraordinary preservation have not been fully constrained. In particular,the necessity of bottom water anoxia, long considered a pre-requisite, has been the sub-ject of recent debate. In this study, we apply a micro-stratigraphical, ichnologicalapproachtodeterminebottom wateroxygen conditionsunder,which BurgessShale-typebiotas were preserved in the Middle Cambrian Spence Shale of Utah. Mudstones of theSpence Shale are characterizedby fine scale (mm-cm)alternation between laminated andbioturbated intervals, suggesting high-frequency fluctuations in bottom water oxygena-tion. Whilst background oxygen levels were not high enough to support continuousinfaunal activity, brief intervals of improved bottom water oxygen conditions punctuatethe succession. A diverse skeletonized benthic fauna, including various polymerid trilo-bites, hyolithids, brachiopods and ctenocystoids suggests that complex dysoxic benthiccommunity was established during times when bottom water oxygen conditions werepermissive. Burgess Shale-type preservation within the Spence Shale is largely confinedto non-bioturbated horizons, suggesting that benthic anoxia prevailed in intervals, wherethese fossils were preserved. However, some soft-bodied fossils are found within weaklyto moderately bioturbated intervals (Ichnofabric Index 2 and 3). This suggests that Bur-gess Shale-type preservation is strongly favoured by bottom water anoxia, but may notrequire it in all cases. h Anoxia, Burgess Shale, Burgess Shale type-preservation, LangstonFormation, Spence Shale Member,Utah.Daniel E. Garson [daniel.garson@gmail.com], Department of Earth Sciences, University ofCalifornia, Riverside, CA 92521, USA; Robert R. Gaines [Robert.Gaines@pomona.edu],Geology Department, Pomona College, 185 E. Sixth St., Claremont, CA 91711, USA; MaryL. Droser [mary.droser@ucr.edu], Department of Earth Sciences, University of California,Riverside, CA 92521, USA; W. David Liddel [dave.liddell@usu.edu], Department ofGeology, Utah State University, Logan, UT 84322-4505, USA; Aaron Sappenfield[aaron.sappenfield@email.ucr.edu], Department of Earth Sciences, University of California,Riverside, CA 92521, USA; manuscript received on 14 September 2010; manuscript acceptedon 04 February 2011.

Recurrent taphofacies, including conservation lagerstatten, are identified within a spectrum of facies in fourth-order sequences in the Middle Cambrian Wheeler, and Marjum formations of the Drum Mountains and House Range, west-central Utah. These sequences are 3–20 m thick and commence with sharply-based compact, oncolitic, oolitic, or pelletal pack- and grainstones with sharply-defined, corroded and mineralized upper contacts that record drowning discontinuities and early transgressive systems tracts (TSTs). Overlying intervals of calcareous shale and thin-bedded wacke- to packstones with abundant, disarticulated polymerid and agnostoid trilobites represent late TSTs. These are commonly followed by lavender-gray mudstones rich in sponge spicules and comminuted fossil debris that reflect condensed maximum flooding zones. The overlying early highstand (HST) intervals of black, fissile shales are typically barren except for indistinct, circular carbonized algae, but in rare instances, include soft-bodied animal remains. A combination of lower dysoxic–anoxic conditions, with a fluctuating oxycline, and relatively rapid episodic influx of fine-grained detrital sediment favored repeated burial and preservation of abundant organic detritus and rarely soft-bodied animals. Interbedded dark gray, shales include abundant articulated agnostoid trilobites and diminutive polymerids (e.g., Jenkinsonia). Overlying platy, calcareous bedding planes covered with articulated bodies and molts of the polymerid Elrathia indicate rapid blanketing of undisturbed seafloors by calcareous mud layers. These beds grade upward successively into interbedded, sparsely fossiliferous platy to flaggy shale and thin, pale gray weathering calcisiltites, and burrow-mottled to nodular limestones, recording late HST to falling stage (FSST) carbonate shedding. Thin calcisiltites include fossil debris and articulated larger polymerid trilobites and the eocrinoid Gogia, preserved by obrutionary deposits. The repeated recurrence of these patterns provides the rudiments of a predictive model that not only explains the differing modes of preservation but may also aid in prospecting for new lagerstatten.

Freshwater mussels and stream substrate were sampled at 30 locations in the Apalachicola, Chattahoochee, and Flint (ACF) river basins. Although >2100 mussels that included 25 species were observed in our sample of 2661 quadrats, only 5 species were sufficiently abundant to provide reliable estimates of the relationship between mussel presence and substrate composition. Among these 5 species, only Villosa lienosa was associated with substrate composition. Villosa lienosa was most prevalent in well-sorted sediments that contained high proportions of fine particles, but its presence was unrelated to sediment porosity. Because many species of freshwater mussels in these Coastal Plain streams are dispersed and rare, future studies of mussel–habitat associations in the ACF basin should include novel sampling designs and methods that allow rare species to be encountered in greater numbers than those observed in our study.

We compare the preservation (taphonomic grade) and age ofChione(bivalve) and foraminifera from modern siliciclastic tidal flat sediments of Bahia la Choya, Sonora, Mexico (northern Gulf of California). Disarticulated shells ofChionecollected from the sediment-water interface of Choya Bay exhibit a substantial range in taphonomic grade and age, several hundred years to ∼80–125 ka based on Accelerator Mass Spectrometer14C dates and D-Alloisoleucine/L-Isoleucine values. There is not, however, a one-to-one correspondence between age and taphonomic alteration ofChione:old (or young) valves may be highly altered or they may be relatively pristine. In contrast toChione, most foraminiferal tests at Choya Bay are quite pristine, which suggests a quite young age, but tests are surprisingly old (up to ∼2,000 calendar years based on Accelerator Mass Spectrometer14C dates).We suggest that following seasonal pulses in reproduction, some foraminiferal tests are rapidly incorporated into a subsurface shell layer by “Conveyor Belt” deposit feeders and preserved there, while the rest of the reproductive pulse rapidly dissolves. Ultimately, some of these buried tests, along withChione, are transported back to the surface by biological activity and storms. The much greater range of taphonomic grades and ages amongChioneshells suggests that they, unlike foraminifera, are sufficiently large and preservable (low surface/volume ratio and chemical reactivity) to undergo many cycles of degradation, burial, and exhumation before complete destruction. The age of foraminiferal tests indicates that time-averaging of microfossil assemblages at Choya Bay is much more insidious than would be expected considering the relatively pristine state of the tests alone.Based on our studies, the lower limit of temporal resolution of shallow shelf microfossil assemblages appears to be ∼1000 years. We caution, however, that each depositional setting (taphofacies) should be evaluated on a case-by-case basis before gross generalizations are made. Indeed, the discrepancy between age and taphonomic grade of fossil assemblages at Choya Bay suggests that neither hardpart size or taphonomic grade are infallible indicators of test preservability or likely temporal resolution of the host assemblage, and that the dynamics of hardpart input and loss must also be evaluated.

Foraminiferal reproduction and preservation have been studied in Holocene tidal flat sediments of Bahia la Choya, Sonora, Mexico (northern Gulf of California). Foraminiferal reproduction at Choya Bay tends to occur in discrete (~ a few weeks) seasonal pulses, which are then followed by periods of homogenization and dissolution of several months duration. Foraminiferal number (number of tests/gram sediment) increases northward across the flat primarily because of decreasing intensity of bioturbation and increasing total carbonate weight percent (shell content) of sediments. Despite intensive dissolution of foraminiferal reproductive pulses, tests which appear to be relatively fresh are actually quite old (up to ~ 2000 years based on 14C dates). We hypothesize that after reproduction some tests survive dissolution because of rapid advection (burial) downward by conveyor belt deposit feeders (e.g., callianassid shrimp, polychaete worms) into a subsurface shell layer, where tests are preserved until exhumation much later by biological activity or storms. Thus, taphonomic grade (surface condition) of foraminiferal tests in these sediments is not an infallible indicator of shell age (time since death). The condition of the test surface is indicative of the residence time of the test at the sediment-water interface (“taphonomically active zone”) and not test age.

Fringing coral reefs at Discovery Bay, Jamaica, exhibit a pronounced depth-related gradient in water turbulence and associated physicochemical taphonomic factors (abrasion, dissolution), and thus provide ideal settings for investigating the influence of taphonomic processes on the formation of fossil assemblages. Foraminifera are prominent constituents of bioclastic sediments at Discovery Bay, and exhibit a high diversity of test sizes, shapes, wall compositions, architectures, and microstructures which may potentially affect their post-mortem behavior. We have developed a taphofacies model for Jamaican north coast fringing reefs and associated environments that has allowed us to generate hypotheses about the formation of foraminiferal sediment assemblages

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 Halimedu (0.4% to 38.7%), coralline algae (4.7% to 16.2%) and the encrusting foraminiferan Homotremu 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 Hulimedu are most abundant in sediments from the back reef and deeper portions of the fore reef (2 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).

Substrata for numerous epizoans in the Silurian Waldron Shale were provided by the toppled and overturned calyces of camerate crinoids. These were inhabited by a skeletonized fauna consisting of at least 25 species; including several bryozoans, tabulates, inarticulate brachiopods, worms, and echinoderms.Many of the encrusting bryozoans exhibit skeletal overgrowths which appear to reflect competitive interactions for space. Coexistence of certain of the bryozoans involved in such interactions is suggested by skeletal features such as inflated growth forms and upturned margins at junctions between colonies (both representing attempts to avoid being overgrown). In addition, the occurrence of overgrowth reversals within individual encounters indicates contemporaneity of that pair. Species may be ranked according to their relative overgrowth success; however, this ranking is far from rigid as lower ranked species may on occasion overgrow more highly ranked species. Such nontransitive patterns appear similar to those described from Recent cryptic bryozoan faunas.

The routine procedure for most distributional studies of foraminifera has been to count approximately 300 specimens of all sizes greater than some specified minimum (usually between 63 and 125 μm), and determine percent abundance of each species using total counts. This method fails to take into account that foraminiferal species may be found predominantly within certain size fractions, either because of species-specific size ranges or taphonomic processes (e.g., sorting, transport, abrasion, dissolution). Use of an alternative counting procedure (“sieve method”) takes into account foraminiferal size distributions. The sieve method utilizes counts of up to 300 specimens in each sand size fraction (0.125–0.25, 0.25–0.5, 0.5–1.0, 1–2 mm) of each sample. Counts are then totaled for each sample (up to 1200 specimens per site) and used in determination of species abundances for each site. The sieve method has been of considerable utility in recognition of foraminiferal bathymetric zonation preserved in sediment assemblages of fringing reef environments at Discovery Bay, Jamaica. Well-documented reef zones (originally based on corals and physiography) are relatively well-defined in Q-mode cluster analysis (UPGMA) of species abundances determined using the sieve method. In contrast, reef zones are not recognized in cluster analysis of foraminiferal species abundances based on the standard method, nor by cluster analysis of species abundances within individual size fractions. The sieve counting procedure compensates for operator bias in specimen counts introduced by large and unusually abundant species (e.g.,Amphistegina gibbosa) and by small but colorful forms (e.g.,Discorbis rosea), which mask the zonation using the standard procedure. The sieve method does not alter overall depth-related trends in species abundance, distribution, or diversity as determined by the standard method. Thus, foraminiferal sediment assemblages contain more paleoenvironmental information than had previously been thought.

Tropical marine communities from shallow-water (

The 125,000 y.b.p. (Sangamon) Falmouth Formation is an emergent fringing-reef complex exposed along the north coast of Jamaica. Q-mode cluster analysis using constituent composition of Falmouth Formation rocks collected near Discoveiy Bay was employed to differentiate two distinctive facies. These are a dense, well-lithified skeletal packstone containing abundant calcareous algae (Halimeda and coralline algae) and molluscs, and a poorly lithified skeletal grainstone composed primarily of sand-sized coral fragments and coralline algae with only minor amounts of Halimeda. The composition of sediments within these facies is comparable to the composition of back-reef and shallow (5-8 m deep) fore-reef sediments of the Holocene Jamaican fringing-reef system. These results contradict the pervasive idea that sandsized, reef-derived sediments are ineffective as environmental indicators owing to post-depositional transport away from the reef system. Petrographic and X-ray analyses of the mineralogy of Falmouth Formnation limestones reveal that back-reef packstones retain much of their original aragonite and high-Mg calcite. In fore-reef grainstones, however, better sorting (increased permeability) results in dissolution and leaching of these metastable phases and reprecipitation of low-Mg calcite. Thus, variability in original sediment texture may create a preservational bias in the fossil record against the more permeable fore-reef deposits.

Recent foraminifera inhabiting Jamaican north coast fringing reefs exhibit variation in distribution patterns that are related to bathymetry and reef morphology. Parallel traverses were run from the back reef (depth 1-4 m) across the reef crest, fore-reef terrace (5-14 m), escarpment (24 m), slope (30-55 m), and upper (75 m) deep fore-reef zones at Discovery Bay, Jamaica. Approximately 150 species belonging to 80 genera were identified. Diversity indices (S, H', J') are lowest on the fore-reef terrace, increase toward the back-reef andfore-reef escarpment and slope, and tend to be highest on the mid-lower slope and deep fore reef. Major operational taxonomic groups of foraminifera (suborders Textulariina, Miliolina, benthic and planktonic Rotaliina) exhibit distinct bathymetric trends, with the Miliolina occurring most abundantly in the back reef, and Textulariina and planktonic Rotaliina being most abundant below 30 m. Benthic rotaliines are prominent across the entire reef environment and may compose >60% of foraminiferal sediment assemblages. Among the largest and most common species, Amphistegina gibbosa (benthic Rotaliina) dominates fore-reef assemblages, whereas Archaias angulatus (Miliolina) is most abundant in back-reef environments. Individual fore-reef zones are poorly delineated in Q-mode cluster analysis of foraminiferal species abundances (2 5% of foraminiferal assemblages at any site) based on standard counting procedures (300 foraminifera from all size-fractions >0.125 mm). Reef zones are, however, clearly defined when the standard countingprocedures are applied to individual size-fractions (1-2, 0.5-1.0, 0.25-0.5, and 0.125-0.25 mm) and species abundances determined from total counts of all four size-fractions combined ('sieve method"). Based on the sieve method, Q-mode cluster analysis of common (3% of foraminiferal assemblages at any site) species delineates back reef, fore-reef terrace, and fore-reef slope and upper (75 m) deep fore-reef zones. The alternative counting procedure appears to compensate for operator bias in specimen counts introduced by large and unusually abundant species (e.g., Amphistegina gibbosa) and by small but colorfulforms (e.g., Discorbis rosea), which appear to mask the zonation when using standard counting procedures.