Tuesday, July 5, 2016

Particulate organic matter as paleocurrent indicators: dispersed fossil wood in Pennsylvania Bluestone

The tag line for this blog describes that posts are about geologic carbon, excluding carbonate and aqueous dissolved organic matter, focusing on sedimentary and metamorphic organic matter (OM=organic matter) and products from fossil fuel resources. I come at this topic from the organic petrology or microscopy methods I use to investigate geologic problems of level of diagenesis/ very-low-grade metamorphism or what assemblages of particulate organic matter can convey about depositional environment or climate. Many applications of or advancements in organic petrology are related to fossil fuel exploration or utilization, but there are other non-fossil-fuel applications of sedimentary OM data in the geologic sciences. However, in my experience and opinion, those are not commonly used, either because organic petrology is not part of the usual geology curriculum, therefore, not well known, or because light microscopy is not "high tech".

One occurrence of sedimentary OM greets me frequently while I am walking the dog. I live in an old neighborhood of lawns, large trees, and pachysandra (my personal trace plant for old neighborhoods); most of the homes were built around 1900. Although numerous sidewalks are now cement, many remain the original large slabs of Pennsylvania Bluestone, some with fossil wood fragments exposed on cut or slabbed surfaces. Pennsylvania Bluestone is a Middle to Upper Devonian feldspathic sandstone of the Catskill delta or Catskill/Pocono clastic wedge, outcropping now in southern New York, northeastern Pennsylvania, and northern New Jersey. It is the "molasse" of the Acadian orogeny, whose thermal and deformational peak in the northern Appalachians to the east (Maine, New Hampshire, Massachusetts, Connecticut) occurred in the Lower Devonian.

Bluestone in disrepair but shows typical sidewalk slab size and thickness. Twenty-pound (9 kg) puppy for scale.

Bluestone derives its name "from a deep-blue-colored sandstone first found in Ulster County, NY" (http://www.endlessmountainstone.com/bluestone/). In Pennsylvania, the focus of the bluestone industry is in Susquehanna County bordering New York state. Other colors include tans, various grays, and lilac/purple. The Endless Mountain Stone Company website (in 2002, Endless Mountain was the "largest  'bluestone' producer in Northeastern Pennsylvania, FCOFG field guide, p. 85*) also describes the quarrying, cutting or slabbing operations that produce stone for sidewalks, paving, building and facing stone. The environment of deposition of quarried stone includes offshore bars, beaches, and tidal interchannels.
Ripple marks (interference ripples?) on bluestone sidewalk slabs.

Ripple marks, in different location than above, on wet sidewalk in street lights at night.
Although the Pennsylvania Bluestone Association states that the stone is "clear of most organic residues", megascopic particulate fossil wood is occasionally visible. A stratigraphic section of one quarry in the 2002 Field Conference of Pennsylvania Geologists guidebook (Figure 79, p. 86) to the bluestone region, marks locations of "carbonized plant fossils" and "plant-bearing ss".
Patio bluestone showing range of color, some ripple marks, and, in slab in foreground, dispersed fossil wood fragments.
Old bluestone sidewalk slab; fossil wood weathered out leaving casts.

Fossil wood in both rippled and non-rippled bed surfaces in recently-quarried bluestone (in re-laid sidewalk using old original slabs and smaller new slabs to replace broken material). Fragments on non-rippled surface (lower left) show a general consistent orientation.
In the photo above, fossil wood, on the non-rippled bedding surface, is generally aligned due to the ancient water flow direction that deposited the layer and acts as a paleocurrent indicator. Such indicators include any elongated particles including graptolites and other fossils, and sedimentary structures such as ripples, crossbedding, and flute casts. However, alignment of linear objects, such as the wood fragments, can only narrow paleocurrent flow direction to two directions 180 degrees apart. Asymmetrical ripples and flute casts are examples of structures that can define a single direction. Below is an example of paleocurrent direction analysis, showing bidirectional results, from measuring orientation of wood fragments in a Devonian shale of the Appalachian/Catskill basin (in Potter and others, 1979, Devonian Paleocurrents in the Appalachian Basin).

Three more examples of oriented fossil wood in Pennsylvania Bluestone. Blue color in bottom two photos due primarily due to time of day, just prior to sunset.
(from Potter and others, 1979, Devonian Paleocurrents in the Appalachian Basin)

Above is the cumulative paleocurrent direction analysis of Appalachian Devonian sediments, including those of the Catskill/Pocono wedge, showing general western flow and deposition due to unroofing/erosion of the lower Devonian Acadian orogenic thermal/deformational axis to the east. Fossil wood fragments (=particulate land plant organic matter), such as that seen in the bluestone, were an important contributor to this data set.

BTW, at the time this post was written, the blog background was an extreme close-up of wood fragments in bluestone (below).

*Catskill delta field guides:
    Facies and Sedimentary Environments of the Catskill System Tract in Central Pennsylvania, Pittsburgh Association of Petroleum Geologists, 2009 http://www.papgrocks.org/PAPGGuidebook_Spring09.pdf
    From Tunkhannock to Starrucca: Bluestone, Glacial Lakes, and Great Bridges in the “Endless Mountains” of Northeastern Pennsylvania, Field Conference of Pennsylvania Geologists, 2009.