Chert, Part II:

Bioclasts and Pyrite

fossil test [124 kb]

"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.

Bioclasts and Pyrite in Lower Devonian Chert from Pennsylvania

Samples 12 and 40 were collected from two of seven key localities in Snyder county, central Pennsylvania. They represent E.N.E.-trending outcrops of cherty strata which are exposed at numerous localities in a 900 km2 study area. The samples are part of a large provenance study (approximately 100 samples for petrography, 120 for multi-element geochemical analyses) designed to test the utility of numerous tools for matching prehistoric lithic artefacts to outcrops, including known quarry sites from which artefacts, hammerstones and chert debitage have been recovered.

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).

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Left: 2. The `spicules' and pyrite in chert from Walnut Acres, sample 40. 80X magnification, long-axis field-of-view 1.4 mm, transmitted, plane-polarized light.

3. Right: Hydrocarbon test, rhombohedral carbonate and pyrite in sample 40. 160X magnification, long-axis field-of-view 0.7 mm, reflected, plane-polarized light.

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4. Left: Hydrocarbon tests in chert sample "2-Float". 80X magnification, long-axis field-of-view 1.4 mm, transmitted, plane-polarized light.

5. Right: Framboidal pyrite, ovoid instead of the more-abundant spheroidal form, sample "2-Float". 80X magnification, long-axis field-of-view 1.4 mm, reflected, plane-polarized light.


A non-technical summary of the bioclasts follows, modified after Wilson (2002). Macrofossils are extremely rare in a suite of 52 hand specimens. In thin section, however, fossil debris is plentiful. Roughly speaking, 10 thin sections in 50 contain macrofossils, most notably in the form of brachiopod shell fragments, 30 display small rounded tests of microfossils, and 35 have elongate siliceous entities described, on simple morphological grounds, as spicules, although they may possibly have alternative identities, such as brachiopod spines. The rounded tests are generally composed of hydrocarbons, while the elongate spicules, which are most often noted in relatively fine (micritic), ferruginous matrices, are polycrystalline aggregates of granular silica with fine carbonate, hydrocarbon, oxides and other minerals. The fine spatial resolution and sensitivity to chemical bonding makes laser Raman spectroscopy the logical tool to examine these features in more detail. The tests are most probably spores from terrestrial plants, though some of the entities in these rocks have been identified as acritarchs and the remains of chitinozoans (B.E. Eley and P.H. von Bitter, Royal Ontario Museum, pers.commun., 2003).

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).

Some Biostratigraphic Lore

Richardson (1969) provided an overview of Devonian biostratigraphy in Europe and north America. In the lower Devonian, the focus of our present interest, the European Gedinnian, Siegenian and Emsian stages are roughly the equivalent of the Helderberg, Deer Park and the lower half of the Onesquethaw stages in the eastern USA, while the middle Devonian Eifelian and Givetian are roughly equivalent to the upper Onesquethaw and the Cazenovia and Tioughnioga stages respectively. Gedinnian spores tend to be very small: a study of 500 specimens from the Welsh Borderland revealed a range of 8 to 62 microns with a mode of just 17.5 microns, while forms >80 µm are not known. Larger forms appear in the Siegenian and Emsian. Some megaspores (>200 µm) occur in the Siegenian, and in the middle Devonian (Eifelian, Givetian) yet larger forms occur, often >200 µm, quite often 300-500 µm.

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.

Selected References

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.

Graham Wilson, posted 04 December 2002, updated 02 July 2003, html updated 09 January 2006

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