"Rock of the Month # 23, posted for May 2003"
1. Hydrocarbon `test' in Devonian chert from Selinsgrove, Pennsylvania. Laser Raman microprobe analysis suggests that the rather well-ordered, highly reflective hydrocarbon forming the 20-micron-thick wall of this microfossil is graphitic, a surprise in a suite of rocks showing few signs of metamorphism (Mary I. Garland, pers.commun., 2003). Small angular carbonate rhombs and tiny, bright pyrite grains are visible in the finely granular, silica- and dolomite-rich groundmass of this black to brownish-black chert. Sample 12, 160X magnification, long-axis field-of-view 0.7 mm, reflected, plane-polarized light.
The laboratory section of the study focused to a large extent on transmitted and reflected-light petrography, and whole-rock and trace-element geochemistry, with the flexibility to test additional mineralogical techniques such as electron microscope / microprobe analysis and laser Raman microprobe spectroscopy. This document highlights the potential for a classic palaeontological approach. Bioclasts, or their remnants, may be identified as; 1) macrofossils such as shell fragments: 2) spores or plankton, commonly 100-300 microns in diameter, replaced by hydrocarbons: 3) spicule-like forms, siliceous with minute inclusions of other minerals: 4) pyritized debris (framboids are common, possibly nucleated on algal or other biogenic detritus).
The fossil content does not immediately identify the cherts as primary siliceous marine sediments: they lack the depositional laminations noted in some Jurassic cherts and they may well be largely replacements of host carbonate rocks, generated during diagenesis and incorporating bioclasts, including spores, plankton and spicules, that sank into the seafloor carbonate ooze as the limestones and dolostones were accumulating. The fossils, regardless of their exact nature, are potentially useful clues for petrographic provenance.
It is presumed that the observed bioclasts are but a fraction of the material available, a selection fortuitously exposed on the surfaces of polished thin sections. Although some of the samples display better than 90 volume percent silica replacement, 19 out of 50 key samples contain an estimated 51-96 volume percent carbonates, such that standard recovery techniques used on small bulk samples should provide ample microfossil suites.
Chert-bearing units are quite common in the Devonian of the region (Harper, 1999; see also Laird, 1932; Parkins, 1977). Provenance studies involving aspects of chert (macroscopic properties, geology, mineralogy, palaeontology, geochemistry) are nothing new, but the fine grain-size, apparent homogeneity and even the physical toughness and chemical resilience of the material all conspire to keep `chert sourcing' a definite challenge (Luedtke, 1992).
Parkin's (1977) study found that the distribution of acritarchs varies greatly in the Onondaga, even in a single sample. All the spores are miospores (<200 µm and often 25-50 µm in diameter). The most abundant were Apiculiretusispora arenorugosa and Calamospora tenuis. In paleoecological terms, Parkins determined that the Bois Blanc sediments were deposited in a quiet, subtidal marine setting, while the Edgecliffe and Clarence members of the Onondaga were laid down respectively in reef and lagoonal settings.
Of particular note is the Shriver chert member of the Old Port Formation, a lower Devonian unit lying between Ridgeley sandstone and Mandata shale, and laterally correlative to Licking Creek limestone. The Old Port is sometimes mapped alongside the Onondaga chert (Bailey and Katz, 2002). The middle Devonian Onondaga Formation outcrops north of the study area, and has been mapped across parts of southwestern Ontario and western New York state. The Onondaga chert may be distinguished from chert of the lower Devonian Bois Blanc Formation based on petrology and microfossil content, including a `preservation ratio' derived from palynomorphs in the cherts (Parkins, 1977). The latter is the number of well-preserved palynomorphs per 1,000 poorly-preserved palynomorphs, organic fragments and sulphide spheres in a given sample. Spherical sulphide grains up to 30 µm in diameter often occur in the cell walls of palynomorphs, and are considered to be the `remains of organic walled microfossils'. Parkins' detailed study identified many species of microfossils (acritarchs and spores) in both the Bois Blanc and Onondaga cherts.
Wall (2000) examined artefacts at the 36Un82 archaeological site in Allenwood, Pennsylvania, which displays evidence of Middle Archaic through Late Woodland occupations. Late Archaic sites in the Susquehanna (West Branch) river valley show a wide range of lithic resources, both primary (Shriver chert) and cobbles (Onondaga chert). The most common artefact material is a grey to black, medium to coarse chert (probably the local Shriver chert).
This particular study did not make extensive use of micropalaeontological procedures, as it was shown that thermal alteration and deformation have obliterated much of the diagnostic value of the fossils. However, this very finding does help to `fingerprint' the local cherts (see the "sequel", below).
This geological, mineralogical and geochemical study of chert outcrops and artefacts was conducted by staff of consultancy A.D. Marble and Company, Inc. and Graham Wilson of Turnstone Geological Services Ltd. in January 2002- June 2003, on behalf of the Pennsylvania Department of Transportation (PENNDOT), Engineering District 3-0.
BAILEY,DN and KATZ,G (2002) What's black and gray and found all over? Chert in the Middle Creek and Penns Creek watersheds in central Pennsylvania. Eastern States Archaeological Federation Conference, manuscript, 11pp.
HARPER,JA (1999) Devonian. In `The Geology of Pennsylvania' (Shultz,CH editor), Pennsylvania Geological Survey / Pittsburgh Geological Society, Spec.Publ. 1, 888pp., 108-117.
LAIRD,HC (1932) The Nature and Origin of Chert in the Lockport and Onondaga Formations of Ontario. PhD Thesis, University of Toronto, 134pp. plus 18 plates.
LUEDTKE,BE (1992) An Archaeologist's Guide to Chert and Flint. Archaeological Research Tools 7, Institute of Archaeology, University of California, Los Angeles, 172pp.
PARKINS,WG (1977) Onondaga Chert: Geological and Palynological Studies as Applied to Archaeology. MSc Thesis, Brock University, 104pp.
RICHARDSON,JB (1969) Devonian spores. In `Aspects of Palynology' (Tschudy,RH and Scott,RA, editors), Wiley-Interscience, New York, 510pp., 193-222.
WALL,RD (2000) A buried Lamoka occupation in stratified contexts, West Branch valley of the Susquehanna River, Pennsylvania. Pennsylvania Archaeologist 70 no.1, 1-44.
WILSON,GC (2002) Geochemistry of chert samples from Snyder and Union counties, Central Pennsylvania. TGSL Report 2002-09 for A.D. Marble and Company, Inc., Conshohocken, PA, 10+30pp.
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