COAL SEAMS - EVIDENCE OF ALLOCHTHONY
Abstract
Field observations of coal seams at a site in the Warrior coal field and
a site in the Cahaba coal field show no evidence that these two seams
were autochthonous swamp deposits; instead, observations point to
allochthonous coal formation. The underclay beneath each coal seam
shows evidence of being a sedimentary deposit which was subsequently
overlain by detritus from floating organic debris (ultimately coal)
which was overlain by silt/sand. Vertical fossil tree trunks found in
both the underclay and the shale/sandstone above the coal seams suggest
sedimentary rates of deposition of relatively short time (several years
or less) for these units adjacent to the coal. Sharp paraconformity
contacts between the coals and underclays and interfingering of coal
with roof rock suggest the coal was formed with the adjacent rocks in a
single transgressive and/or regressive sedimentary event.
Introduction
Coal seams are commonly interpreted as being formed from ancient swamp
(autochthonous - organic material deposited in situ) deposits in tidal
flat areas. The primary evidence cited to support this interpretation
is the existence of tree roots and rootlets preserved intact and
crosscutting undeformed bedding. Rootlets are said to be too delicate
to have survived uprooting, transportation, and reburial. Rootlets
which crosscut undeformed bedding are said to be in situ because bedding
would be deformed if the rootlets had been floated in a mat of
vegetation and pressed into soft mud as the mat was grounded in shallow
water. [1] Therefore, the evidence is generally said to support the
theory that coal seams are swamp deposits.
Present-Day Swamp Features
Present-day swamps show a density of standing trees which generally
interrupt line-of-sight in any horizontal direction within a distance of
less than 200 to 300 feet from the observer. Organic matter from the
trees collects on the swamp floor and is preserved in a reducing
depositional environment. The tree stumps interrupt the horizontal
continuity of the organic mat and roots/rootlets from each stump
penetrate into the soil below. If the swamp was located in a tidal-flat
area, buried by a storm-driven wash-over of sediment, and eventually
preserved in the stratigraphic record now observable in outcrop, we
would predict the presence of certain features. A cross-section through
this hypothetical swamp deposit would show the features depicted in
Figure 1. We would expect to see tree stumps in the coal with attached
roots/rootlets penetrating the underlying strata. Many of these stumps
would have attached trunks entombed in the lithified wash-over of sand
and silt. If the tree stumps had been uprooted during the wash-over, we
would see potholes left by the uprooted stumps which had been filled
with the overlying sediment. The organic layer and underlying soil
would be intensely penetrated by roots, leaving little or no bedded
structure and creating a transitional zone between the underclay and
coal.
Common Features of the Subject Coal Seams
Figure 2 is a generalized cross-section of features observed in outcrops
of the Warrior Basin Jefferson coal seam just northeast of I-65's Exit
282 near Warrior, and the Cahaba Basin Nunnally Group coal seam
one-quarter mile south of the junction of Highways 150 and 31 in Hoover
(just south of Birmingham). Very thin-bedded structures are visible in
the coals, and thin layers of impurities may commonly be traced for tens
of feet along an outcrop. Stumps are typically absent from coal seams,
although the mold of a stump was observed (without attached roots) in
the middle split of clay between the upper and lower beds of the
Jefferson coal. Organic fragments are common in the underclay which
consists of a gray, silty thin-bedded clay. The overlying rock consists
of gray shale, siltstone and sandstone with fossil plant fragments. The
contact of the coal with the underclay is sharp rather than gradational.
Of several hundred feet of outcrop, one vertical fossil tree trunk was
found just above the Jefferson coal seam (see Photograph). The base of
the tree appears to have been severed at roughly 60 degrees and displays
no roots. No root systems were observed in the sedimentary strata
above, in or below the Jefferson or Nunnally seams. Another vertical
fossil tree trunk was observed in the underclay beneath the Nunnally
Group coal seam. The top of the tree trunk did not penetrate the coal;
the base of the tree is still buried in the bank of the outcrop.
Undeformed thin horizontal bedding is terminated abruptly by both
vertical fossil tree trunks, one above the Jefferson and one below the
Nunnally coal seams.
Interpretation
The vertical fossil tree trunks above and below the coal seams must have
been buried rapidly (within a few years or less), before the trunks had
time to disintegrate. Because the crosscut bedding is not deformed, the
trees did not sink from a floating mat of vegetation to be pushed into
the soft sediment, and because at least one of the trees has no attached
roots, the tree(s) were not buried in situ but rather floated into a
vertical position, sank to the bottom, and were rapidly buried. By
analogy, we can infer that root systems which crosscut undeformed
bedding, as described in Reference 1 regarding a locality near
Brooksville, Alabama, may have also floated to the bottom of a
depositional basin and been buried rather than having grown in situ (as
postulated by the Reference 1 author). Root systems crosscutting
bedding in Figure 3 (located near Brooksville) of Reference 1 appear to
be detached from stumps and may exhibit a preferential alignment -
characteristics cited by Rupke as indicators of rapid burial in flowing
water. Rupke also cited filling of root systems with sediment unlike the
enveloping rock as another indicator of rapid burial in flowing water.
[2]
Schultz analyzed 400 samples from ten underclay zones located in the
Appalachian, Mid-Continent and parts of the Illinois basins by X-ray
diffraction, and concluded that the mineralogy of the underclays did not
reflect degradation by weathering, as would be expected if the underclay
had been a topsoil. Field observations by Schultz showed "·that
underclays were formed before deposition of coal-forming material began
and therefore cannot be the residual soils on which the coal-forming
flora grew." Schultz therefore concluded that the underclays were not
residual soils which weathered in situ, but rather the result of
transportation and flocculation of clay. [3] Field observations from the
present study suggest Schultz' conclusions may also apply to the
Jefferson and Nunnally underclays.
The absence of in situ stumps (stumps with attached roots) and/or
potholes from uprooted tree stumps in the coal/underclay; the
thin-bedded structure of the coal/underclay; the sharp paraconformity
between the coal and underclay; thin beds of clay or other impurities
within the coal seams which extend uninterrupted by vertical tree
stumps, trunks, intact root systems, or potholes for hundreds of feet;
the lack of observed root penetrations in either the coal or underclay;
interfingering of the coal with overlying clastics; and vertical fossil
tree trunks which crosscut bedding above and below the coal collectively
lend support to the allochthonous model of coal seam formation for the
Jefferson and Nunnally Group coal seams.
Conclusion
The observations listed above suggest that autochthonous interpretations
of the Jefferson and Nunnally Group coal seams should be re-examined.
This author predicts that observations of coal seams visited on our
October 11 field trip will display allochthonous characteristics similar
to those described above for the Jefferson and Nunnally coal seams.
William W. Payne, P.G.
Law Engineering and Environmental Services, Inc.
2100 RiverChase Center, Suite 450
Birmingham, Alabama 35244
Telephone: 205-733-7624
Facsimile: 205-985-2951
bpayne@voyageronline.net
REFERENCES
1. GASTALDO, Robert A. 1984, Reprinted 1996. A Case Against
Pelagochthony: The Untenability of Carboniferous Arborescent
Lycopod-Dominated Floating Peat Mats. The Paleontological Society
Special Publication No. 1: pp. 97-116.
2. RUPKE, N. A., 1969, Sedimentary evidence for the allochthonous
origin of Stigmaria, Carboniferous, Nova Scotia: Geological Society of
America Bulletin, vol. 80, pp. 2109-2114.
3. SCHULTZ, Leonard G. 1958. Petrology of Underclays. Bulletin of
the Geological Society of America, Volume 69: pp. 363-393.