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Data from: Palynology of a short sequence of the Lower Devonian Beartooth Butte Formation at Cottonwood Canyon (Wyoming): Age, depositional environments and plant diversity

Citation

Tomescu, Alexandru; Noetinger, Sol; Bippus, Alexander (2021), Data from: Palynology of a short sequence of the Lower Devonian Beartooth Butte Formation at Cottonwood Canyon (Wyoming): Age, depositional environments and plant diversity, Dryad, Dataset, https://doi.org/10.5061/dryad.zcrjdfnb2

Abstract

The Beartooth Butte Formation hosts the most extensive Early Devonian macroflora of western North America.  The age of the flora at Cottonwood Canyon (Wyoming) has been constrained to the Lochkovian-Pragian interval, based on fish biostratigraphy and unpublished palynological data.  We present a detailed palynological analysis of the plant-bearing layers at Cottonwood Canyon.  The palynomorphs comprise 32 spore, five cryptospore, two prasinophycean algae and an acritarch species.  The stratigraphic ranges of these palynomorphs indicate a late Lochkovian - Pragian age, confirming previous age assignments.  Analyses on samples from three different depositional environments of the plant-bearing sequence – layers with in situ lycophyte populations, flood layers that buried those populations and an organic matter accumulation zone within a flood layer – demonstrate distinct palynofacies. Comparisons between palynomorph and plant macrofossil diversity reveal some discrepancies.  Whereas zosterophylls and lycophytes, most diverse and abundant among the macrofossils, have only one known corresponding spore type (assignable to zosterophylls) in the palynomorph assemblage, the trimerophytes, rare in the macrofossil assemblage, are represented by three spore types.  Some of these discrepancies reflect taphonomic differences between macrofossils and palynomorphs, others could be due to the fact that the parent plants of most palynomorph types in the Cottonwood Canyon assemblage are unknown.  These observations emphasize the need for concerted efforts to bring together the knowledge of macro- and microfloras within Early Devonian localities.  Nevertheless, given the palaeophytogeographic significance of the Beartooth Butte Formation flora, its palyno- and macrofossil assemblages, taken together, provide new data relevant to future discussions of Early Devonian biogeography.

Methods

Data collection, generation, and processing.

Geology
The Beartooth Butte Formation is a discontinuous Lower Devonian unit bounded by the Ordovician Bighorn Dolomite and the Upper Devonian Jefferson Dolomite (or Jefferson Dolomite).  The unit consists primarily of dolomitized siltstone and shale with dolomitized sandstone interbeds.  The large-scale geometry of the Beartooth Butte Formation, its sedimentology and fossil content, along with isotopic data, suggest fresh- to brackish-water depositional environments of estuarine to fluvial nature.  Aside from the type locality at Beartooth Butte (Park County, Wyoming), the unit is exposed at a few other known locations throughout the rugged landscape of northern Wyoming and southern Montana.  One of the best studied of these is the Cottonwood Canyon locality, in the Bighorn Mountains of northern Wyoming, Big Horn County.  Analyses of the palynoflora at the two localities by D.C. McGregor proposed a late Lochkovian to early Pragian age for the Beartooth Butte Formation at Cottonwood Canyon, and a mid-late Emsian age at the Beartooth Butte locality.  These age assignments are broadly consistent with fish biostratigraphy data.

Depositional environments and sampling
A total of 28 palynological samples were collected from different horizons of a c. 1-meter thick sequence of the Beartooth Butte Formation at Cottonwood Canyon.  The sampled horizons, rich in plant macrofossils, are part of a heterolithic sequence consisting of an alternation of two types of deposits.  One type is represented by dark, finely laminated shales that preserve dense in situ populations of the early lycopsid Sengelia radicans, which formed dense mats of interwoven stems with rhizomatous growth.  The Sengelia shales alternate with massive beds of hard-cemented siltstones rich in transported plant material and other organic detritus, often highly fragmented. The siltstone beds are penetrated by vertical in situ root-bearing axes from the Sengelia populations of the overlying dark shale layer.  The heterolithic sequence has been interpreted as reflecting cycles of colonization by Sengelia of flood deposits at the water’s edge, and subsequent burial of Sengelia populations by periodic flood events.
Samples were processed at the Laboratory of Palaeopalynology (Palaeontology Section, Museo Argentino de Ciencias Naturales “Bernardino Rivadavia” – MACN) following conventional methods with HCl and HF acid maceration and they were not oxidized.  Extracted organic residues were sieved through a 10 µm mesh and the >10 µm fraction was mounted on standard microscope slides with acrylate.  Productive samples were analysed and imaged for identification of palynomorphs and quantification of palynofacies,  The material (slides, residues, rock samples) is deposited in the MACN collections under BA Pal accession numbers 6615-6641.

Palynofacies analysis
To explore the correlation between depositional environments and palynofacies, a subset of 11 samples differentiated between the two types of sediment – Sengelia shales and flood deposits, referred to as “Sengelia mat” (3 samples) and “flood layer” (8 samples), respectively – were studied.  Aside from these two types of sediment, an especially thick flood layer hosted a thin (<1 cm thick) but conspicuous lens of dark material covering a c. 2 m2 surface area.  This dark lens consisted of a concentrated accumulation of mostly fragmentary and amorphous organic matter, with a significant plant fraction.  This lens rich in organics was sampled separately and is referred to as “organic lens” (2 samples).
In each of the samples analysed for palynofacies two tallies were performed under transmitted light microscopy.  To ensure consistency of tallies between samples, we mounted equal amounts of organic aliquot extracted from each sample on slides, diluting it with the same amount of mounting medium.  To account for inherent variability in the distribution of palynofacies elements in the slides, we tallied all palynofacies elements (main axis >5 µm) observed in the field of view at 200x total magnification in several randomly selected areas in each slide, until reaching a target minimum of 100 observations per sample and, where possible (in most cases, i.e., 10 out of 13 samples), more than 300 observations.
Preliminary test-tallies led us to distinguish the following categories of palynofacies elements for the analysis (Fig. 3): amorphous organic matter (AOM); wefts of fibrous structures (WFS); structured phytoclasts, translucent, sheet-like, showing cell outlines (SPCO); structured phytoclasts, mostly brown to dark brown, often with opaque areas and usually with sharp edges (SPSE); phytodebris, usually brown, with less well defined surface structures or edges (PLD); phytodebris with well-defined round outline (PDRO); opaque organic debris, black, mostly of small size (OOD); and spores (SPO).

Parent plants of dispersed spores
The Beartooth Butte Formation hosts a relatively diverse, albeit incompletely characterized, macroflora at Cottonwood Canyon.  To explore correlations between this macroflora and the palynoflora, we queried the literature to assemble a list of parent plant species for those spores and cryptospores in our samples for which this type of data have been published.

Funding

National Science Foundation, Award: 1546593,IIA‐1322504

Humboldt State University

American Philosophical Society

Consejo Nacional de Investigaciones Científicas y Técnicas, Award: PIP 11220120100182CO