Detrital Zircon Evidence of a Recycled Orogenic Foreland Provenance
for Alleghanian Clastic-Wedge Sandstones
William A. Thomas, Thomas P. Becker, Scott D. Samson,1 and Michael A. Hamilton2
Department of Geological Sciences, University of Kentucky, Lexington, Kentucky 40506-0053, U.S.A.
(e-mail: [email protected])
ABSTRACT
Late Paleozoic synorogenic clastic wedges in the Appalachian foreland basin contain erosional detritus from the
Alleghanian orogenic belt; however, the relative importance of detrital contributions from the frontal orogen, the
orogenic hinterland, and the distal craton has not been resolved. To evaluate these components, we analyzed detrital
zircons from two sandstones in the Pennington-Lee clastic wedge in eastern Tennessee: the Lower Pennsylvanian
Sewanee Conglomerate and the Middle Pennsylvanian Cross Mountain Formation. Ages of detrital zircons are similar
in the two sandstones, ranging from 365 to 2860 Ma. The age range spans the Acadian and Taconic orogenies, the
Grenville province, and pre-Grenville rocks (Granite-Rhyolite province, midcontinent orogens, and Superior province)
of the Laurentian craton. The Acadian-, Taconic-, and Grenville-age detrital zircons have sources in Alleghanian
frontal crystalline thrust sheets. The pre-Grenville zircons indicate an original source in the Laurentian craton (Canadian shield), and a longitudinal river system headed in the shield might have supplied detritus directly to the
Appalachian foreland basin in Tennessee. Alternatively, all of the Proterozoic and Archean ages of zircons represented
in the Sewanee and Cross Mountain sandstones have been identified in the detrital zircon populations of preorogenic
Iapetan synrift and passive-margin strata along the eastern Laurentian margin, suggesting a source of recycled zircons
from preorogenic rocks exposed in Alleghanian thrust sheets. A lack of detrital zircons contemporaneous with the
Alleghanian orogeny shows that Alleghanian igneous rocks had not been integrated into the drainage network and/
or had not been exhumed. The age distribution of the detrital zircons suggests deformation and erosion of rocks of
the Laurentian margin and emphasizes the importance of preorogenic architecture of the continental margin in the
construction and erosion of the orogenic belt.
Online enhancement: table.
Introduction
Tectonic models for orogenic belts mechanically
link emplacement of a tectonic load to subsidence
of a lithospheric plate, forming a foreland basin
(e.g., Jordan 1995). The tectonic load integrates
preorogenic rocks of the continental margin with
a subduction zone, possibly including a volcanic
arc complex and accretionary prism, frontal thrust
sheets of preorogenic stratigraphic cover and continental basement, older continental-margin orogenic belts, and accreted exotic terranes. All of the
disparate rocks in the tectonic assemblage potentially contribute to erosional detritus that is de-
posited in a foreland basin (e.g., Johnson and Beaumont 1995). Distributions and gradients of foreland
sedimentary thickness, facies and depositional systems, and paleocurrents generally confirm dispersal
of sediment from the tectonic load to the foreland
basin (e.g., Thomas 1977; Jordan 1981, 1995). In
actively growing mountain ranges, headward erosion across structural barriers extends transverse
drainages deeper into the hinterland (Oberlander
1985). Simple proximity suggests that frontal thrust
sheets (adjacent to the foreland basin) should dominate the sources of sediment delivered through
transverse drainages. In addition to the supply of
detritus from the orogenic belt to the foreland,
some sediment may be supplied from the craton
(e.g., Archer and Greb 1995; Robinson and Prave
1995). The extent to which foreland drainage in-
Manuscript received January 23, 2003; accepted July 1, 2003.
1
Department of Earth Sciences, Syracuse University, Syracuse, New York 13244, U.S.A.
2
Geological Survey of Canada, 601 Booth Street, Ottawa,
Ontario K1A 0E8, Canada.
[The Journal of Geology, 2004, volume 112, p. 23–37] 䉷 2004 by The University of Chicago. All rights reserved. 0022-1376/2004/11201-0002$15.00
23
24
W. A . T H O M A S E T A L .
corporates the orogenic hinterland and the proportion of distal craton-derived detritus remain unresolved problems.
To evaluate the relative contributions of the
frontal orogen, the orogenic hinterland, and the
distal craton, we analyzed detrital zircons from
two Pennsylvanian-age sandstones within the Alleghanian foreland basin in a semicircular clastic
wedge centered on the Tennessee salient of the Appalachian thrust belt (fig. 1). The sample locations
are near the axis of the foreland basin within an
inferred drainage system from the orogen (fig. 1) and
are also generally within the course of an inferred
longitudinal (Amazon-scale) river with headwaters
in the Laurentian (Canadian) shield and/or northern Appalachians (Archer and Greb 1995). Possible
sources of detritus from the frontal thrust sheets
of the orogenic belt include Grenville-age (900–
1200 Ma) basement, preorogenic synrift rocks
(∼530–620 Ma and ∼750 Ma), passive-margin
(Cambrian-Ordovician) cover, and remobilized synorogenic and postorogenic sedimentary rocks from
the Taconic (∼440–490 Ma) and Acadian (∼350–420
Ma) orogens. Potential sources farther into the hinterland include volcanic, plutonic, and metamorphic rocks associated with the Alleghanian orogenic event (∼270–330 Ma) as well as accreted
exotic terranes of various ages. In contrast to the
ages of potential sediment sources within the Alleghanian orogen, sources of detrital zircons from
the Laurentian craton are substantially older (Hoffman 1989), most commonly in the age ranges of
1300–1500 Ma (Granite-Rhyolite province), 1600–
1900 Ma (several orogens in the midcontinent), and
2600–2800 Ma (Superior province).
Detrital Zircons in Pennsylvanian
Sandstones in Tennessee
Setting of Sample Sites. Ages of detrital zircons
from sandstone samples of a Pennsylvanian-age
synorogenic clastic wedge in the foreland of the
Alleghanian thrust belt in eastern Tennessee (fig.
1) are used here to identify possible sediment
Figure 1. Map of Alleghanian orogen and foreland, showing location of sample sites of sandstones from which
detrital zircons were analyzed. The map schematically shows the extent of Alleghanian and Ouachita clastic wedges,
thrust-belt salients, thrust boundaries, external basement massifs, the Maritimes extensional basin, and Alleghanian
plutons. Approximate locations of accreted terranes are shown by labels in italics. Sediment dispersal directions
inferred from facies distribution and onlap/downlap patterns are shown for the foreland basins (summary in Thomas
1977). Paleocurrent directions (Meckel 1967; Robinson and Prave 1995) are shown for the Sharp Mountain Member
of Pottsville Formation (S.M.), Olean Conglomerate (O.), and Sharon Formation (S.); correlations are shown in figure
2.
Journal of Geology
ALLEGHANIAN OROGENIC DETRITAL EVIDENCE
sources. Samples were collected from a sandstone
(Lower Pennsylvanian Sewanee Conglomerate,
depositional age ∼316 Ma) near the base of the
Pennsylvanian succession and from the stratigraphically highest preserved sandstone (Middle Pennsylvanian Cross Mountain Formation, depositional
age ∼305 Ma) in the area (fig. 2; tables 1, 2; table 2
is available in the electronic edition of the Journal
of Geology and also is available from the Journal’s
Data Depository in the Journal of Geology office
upon request). The depositional ages are based on
chronostratigraphic correlations in McKee and
Crosby (1975) and the time-scale calibration of Ross
and Ross (1987). A tonstein in the Fire Clay coal is
dated at 311.2 Ⳳ 0.7 Ma (40Ar/39Ar; Lyons et al.
1992), and the Fire Clay coal is correlated with the
Windrock coal (Wanless 1975), which is stratigraphically between the Sewanee and Cross Mountain sample horizons in Tennessee (fig. 2).
The sampled sandstones are part of the Pennington-Lee clastic wedge, which has a semiradial distribution centered on the Tennessee salient of the
25
Alleghanian thrust belt (fig. 1; Thomas 1977). Distribution of deltaic and barrier-island facies generally indicates northwestward progradation (Englund 1974). Quartzose sandstones of the lower Lee
Formation intertongue with marine fine-grained
clastic rocks of the Pennington Formation and prograde both northeast and southwest along present
structural strike (fig. 2) as well as northwest across
strike (Englund 1964, 1968; Englund and DeLaney
1966), suggesting broadly semiradial dispersal from
a provenance southeast of the present thrust-belt
salient (Thomas and Schenk 1988; Hatcher et al.
1989). Sandstone petrography from the northern
part of the same clastic wedge (the Pocahontas,
New River, and Kanawha succession in southern
West Virginia; fig. 2) is consistent with unroofing
of a sedimentary, volcanic, and low-grade metamorphic cover to expose higher-grade metamorphic
and plutonic rocks (Davis and Ehrlich 1974;
O’Connor 1988).
Methods. Two 5-kg sandstone samples were collected from the least weathered parts of the out-
Figure 2. Diagrammatic stratigraphic cross section and chronostratigraphic chart showing correlations between our
sample locations in Tennessee (tables 1, 2) and previously reported samples in Pennsylvania (Gray and Zeitler 1997).
Correlations are from McKee and Crosby (1975), Milici et al. (1979), and Patchen et al. (1984a, 1984b); names of
stratigraphic subdivisions between the sample horizons in Tennessee are not shown here. Stratigraphic positions of
detrital zircon samples are shown by asterisks. Bold dashed line shows generalized level of present erosion surface.
26
W. A . T H O M A S E T A L .
Table 1. Apparent Ages of Detrital Zircons from Two
Alleghanian Sandstones in Eastern Tennessee
Sewanee
Conglomerate
Ma
384.8
396.8
405.2
414.5
435.4
445.3
455.1
459.6
462.9
968.9
985.7
1008.1
1014.7
1027.2
1029.7
1059.1
1030.8
1064.6
1083.6
1085.2
1094.5
1105.0
1144.3
1130.6
1175.6
1214.1
1232.9
1258.2
1312.8
1315.0
1510.8
1600.7
1740.9
1778.3
1783.1
1816.4
1817.1
2121.9
2689.9
2712.0
2753.5
Cross Mountain
Formation
Ⳳ1j
a
4.4
4.8a
5.1a
5.0a
10.4a
5.2a
5.9a
8.1a
4.7a
59.9
16.2a
27.8
11.5a
14.6a
22.5
18.8
14.2a
6.1
17.6
24.4
14.0a
43.5
4.4
14.4
15.3
52.1
31.7
17.6a
14.0
9.4
4.2
8.8
6.9
7.6
18.2
20.7
2.9
10.7
8.2
49.3
8.7
Ma
Ⳳ1j
366.0
384.3
394.8
403.9
413.3
413.8
421.0
428.0
472.3
769.5
888.8
1013.1
1023.0
1026.7
1028.5
1034.5
1035.1
1040.4
1055.5
1067.5
1075.5
1099.0
1154.4
1217.0
1342.7
1362.8
1376.4
1451.2
1567.0
1664.0
1734.4
1742.0
1787.6
1798.1
2687.2
2737.6
2857.7
5.0a
4.7a
9.6a
4.0a
5.5a
4.8a
8.4a
5.3a
5.2a
7.7a
23.1
10.4
51.5
24.3
25.1
19.6
27.4
13.0
6.2
12.2
14.3
22.2
15.6
24.2
20.3
35.2
17.4
24.5
5.5
24.1
16.4
8.6
23.3
48.3
7.0
9.8
4.1
color and morphologic category to minimize possible omission of zircons from rare age groups; however, more zircons were selected from the most
populous color/morphologic groups. With our sampling protocol and the number of zircons dated
from each sample (tables 1, 2), omitting any significant age groups is unlikely.
The zircons were analyzed using the SHRIMP II
facility at the Geological Survey of Canada, Ottawa, following the procedures described in Stern
(1997). Initial Pb isotopic compositions for zircons
older than 800 Ma were estimated using the conventional 204Pb correction method, whereas isotopic ratios for zircons !800 Ma were corrected using both the 204Pb and 207Pb methods (e.g., Stern
1997). For most younger zircons (!800 Ma), the two
correction methods produced dates within error of
each other, and thus only the 207Pb-corrected values
are reported here.
Results. Detrital zircon dates from the stratigraphically lowest sandstone (Sewanee Conglomerate; fig. 2) cluster at 385–415, 435–465, 900–1150,
1700–1820, and 2690–2755 Ma and are scattered
sparsely at 1150–1320, 1510, 1600, and 2120 Ma
(figs. 3, 4; tables 1, 2). Detrital zircon dates from
the stratigraphically highest sandstone (Cross
Mountain Formation; fig. 2) cluster at 365–430,
1000–1100, 1340–1380, 1550–1820, and 2685–2860
Ma and are scattered at 470, 770, 900, 1150–1200,
and 1450 Ma (figs. 3, 4; tables 1, 2). A crude correlation between age and color/morphology of the
Note. Apparent ages are 207Pb/206Pb ages calculated using the
204
Pb method of common Pb correction, except where marked
with a footnote.
a
Ages are 206Pb/238U ages calculated using the 207Pb method.
crops, and both were thoroughly washed before
crushing/disk milling. Separations used disposable
nylon mesh sieves and doubly filtered heavy liquids
to minimize potential laboratory contamination.
To avoid bias in the bulk zircon population, no
magnetic separation was used (Sircombe and Stern
2002). For random selection, if ≥ 30 zircons are analyzed, sampling an age group that represents at
least 10% of the population has a probability of
195% (Dodson et al. 1988). Rather than random
selections, our selection included each general
Figure 3. Concordia diagram of SHRIMP U-Pb isotopic
data for zircons from two Pennsylvanian-age sandstones
in Tennessee (tables 1, 2). Ellipses represent 1j uncertainty for analyses of each zircon grain. Insets A and B
show enlargements of part of the plot.
Figure 4. Diagram showing time correlation of detrital zircon ages, depositional ages of sampled sandstones, and times of orogenic events. Ages are plotted
only for zircon dates with !5% uncertainty (except 486 Ⳳ 51 Ma zircon in Blaylock is shown).
28
W. A . T H O M A S E T A L .
zircons in both sandstones shows that round, pink
zircon grains are 11000 Ma; well-faceted yellow
grains are generally younger. These zircon populations indicate no significant change through time,
suggesting persistence of provenance character and
sediment-dispersal systems during the ∼11 m.yr.
spanned by the depositional ages of the two sandstones. The inferred crystallization ages of the detrital zircons span a range that includes the Acadian
and Taconic orogens and Grenville province as well
as the Granite-Rhyolite province, “midcontinent”
orogens, and Superior province of the Laurentian
craton (figs. 4, 5). No detrital zircons have crystallization ages contemporaneous with Alleghanian
orogenic events, and only one detrital zircon might
represent a non-Laurentian accreted terrane.
Discussion of Possible Sediment Sources for
Detrital Zircons in Pennsylvanian-Age
Sandstones
Pre-1300-Ma Zircons. The oldest detrital zircons
in the Alleghanian sandstones correspond in age to
the Superior province (2600–2800 Ma) in the Laurentian shield and to the ∼1600–1900-Ma “midcontinent” orogens (Torngat, Trans-Hudson, Penokean, Yavapai, Mazatzal, and Central Plains
orogens) and Granite-Rhyolite province (1300–1500
Ma) around the central shield (figs. 4, 5). Although
the ages of the detrital zircons document an original source in the craton, the alternatives of direct
supply from a primary craton provenance or of recycling from older craton-derived sediments along
the pre-Appalachian Laurentian margin are not yet
resolved.
Grenville Province. Grenville-age zircons comprise a substantial component of the detrital zircon
population in our Pennsylvanian-age sandstone
samples. Metamorphic and plutonic rocks of the
Grenville orogenic belt define assembly of the Rodinia supercontinent at ∼1200–900 Ma (e.g., Dalziel
1997). The metamorphic terranes encompass older
rocks ranging to 1700 Ma; however, these are more
common in the north (Labrador) than farther south
(Cawood and Nemchin 2001). The subsequent
breakup of the supercontinent left a band of rocks
of the Grenville province as part of the continental
“basement” along the eastern rim of Laurentia (fig.
5). Grenville rocks are incorporated in Appalachian
external and internal basement massifs (fig. 1), and
the precursors of the present eroded massifs may
have been primary sources of detritus supplied directly to the foreland basin. Alternatively, Grenville rocks presently are exposed around the southeastern margin of the Laurentian shield.
Synrift Rocks. The Mount Rogers volcanic rocks
(fig. 5) and related plutons with ages of ∼750 Ma
suggest an early rift event (Su et al. 1994; Aleinikoff
et al. 1995); however, the extent and nature of the
rift are uncertain. These igneous rocks are a potential source of ∼750-Ma zircons, which are represented in our samples of the Alleghanian clastic
wedge.
Diachronous rifting and opening of the Iapetus
Ocean are documented by igneous rocks that range
from ∼620 Ma in Newfoundland to 530 Ma along
the Southern Oklahoma fault system (fig. 5; Hogan
and Gilbert 1998; Cawood and Nemchin 2001).
Synrift igneous rocks (572 Ⳳ 5 to 564 Ⳳ 9 Ma) confirm the age of the Blue Ridge rift (fig. 5; Aleinikoff
et al. 1995; Walsh and Aleinikoff 1999).
Thick, laterally variable accumulations of sediment record Iapetan rifting along the Laurentian
margin (summary in Thomas 1991). Analyses of
detrital zircons from Appalachian synrift sediment
presently are available only from Newfoundland,
where Cawood and Nemchin (2001) analyzed three
samples. Detrital zircon ages of 572–628 Ma (fig. 4)
are roughly synchronous with the depositional age
of the late Precambrian synrift strata, indicating
local synrift igneous sources of sediment. A detrital
zircon age of 760 Ⳳ 40 Ma contrasts with the ages
of nearby synrift igneous rocks but is coeval with
the older volcanic rocks at Mount Rogers, possibly
representing axial sediment transport (Cawood and
Nemchin 2001) or reworking of a local ash deposit.
The synrift samples are dominated by Grenville
(999–1165 Ma) and older zircons with age clusters
at 1230–1360, 1740–1870, and 2600–2890 Ma (fig.
4). Proximal rift shoulders are the likely sources of
Grenville detritus, but the older detrital zircons require sediment dispersal from the distant Laurentian shield (fig. 5; Cawood and Nemchin 2001). Two
of the synrift samples contain similar zircon populations of a wide range of older ages; however, one
sample lacks zircons older than Grenville (Cawood
and Nemchin 2001), suggesting variations in the
location of drainage systems and dispersal points
into the rift through time. Later reworking of the
synrift deposits constitutes a potential local supply
of detrital zircons that represent all of the components of the Laurentian craton.
Passive-Margin Strata. A basal sandstone records
a diachronous transition from synrift strata to
passive-margin deposits along the Laurentian Iapetus margin (fig. 6; e.g., Thomas 1991; Cawood et
al. 2001); initial transgression of the basal passivemargin sandstone ranges in age from earliest Cambrian (543 Ma) to early Late Cambrian. The passive-margin shelf-carbonate succession includes
Figure 5. Map of Laurentian continental crust including provinces of basement rocks from the shield to the Grenville
province (generalized from Hoffman 1989 and Van Schmus et al. 1993), combined with map of outline of late
Precambrian–Early Cambrian rifted margin (Thomas 1977, 1991), showing inferred dispersal of sediment to synrift
deposition. Map shows present and palinspastic locations of synrift, passive-margin, and pre-Alleghanian synorogenic
rocks from which detrital zircon ages are available (as listed in fig. 4).
30
W. A . T H O M A S E T A L .
Figure 6. Hypothetical cross section to illustrate the distribution of detrital zircons in synrift and passive-margin
deposits. Grenville basement was progressively onlapped and covered by sediment before deposition of the younger
strata of the passive margin.
quartzose sandstone interbeds (e.g., Read in Rankin
et al. 1989), which might represent reworking of
the basal sandstone or sedimentary transport of detritus from the center of the craton onto the shelf.
The basal (rift-drift transition) sandstone (Bradore
Formation) in Newfoundland contains detrital zircons exclusively of Grenville age (figs. 4–6; Cawood
and Nemchin 2001), indicating a local Grenville
basement source or reworked synrift sediment. The
basal sandstone (Poughquag Quartzite) in New
York is dominated by Grenville-age zircons, but the
sample also contains zircons with dates of 1300–
1540 Ma, suggesting detritus from either older enclaves within the Grenville orogen or the midcontinent Granite-Rhyolite province (fig. 4; McLennan
et al. 2001). The Poughquag Quartzite contains zircons with dates of 643 and 547 Ma, corresponding
to the ages of synrift igneous rocks. Two older zircons (1620 and 1670 Ma) may have been supplied
directly from the craton or recycled from synrift
sedimentary rocks.
Within the passive-margin carbonate-shelf succession in Newfoundland, clastic interbeds (Hawke
Bay Formation) contain detrital zircons with a wide
range of ages (figs. 4–6; Cawood and Nemchin
2001). In addition to Grenville-age (987–1109 Ma)
zircons, the sandstone contains three older clusters
of ages at 1229–1360, 1780–1860, and 2670–2790
Ma, which were interpreted to represent older
Grenville components, the midcontinent orogens,
and the Superior province, respectively.
Off-shelf deep-water passive-margin mud-domi-
nated deposits border the Laurentian passivemargin carbonate-shelf facies. The off-shelf facies
in the Ouachita thrust belt includes the Lower Ordovician Blakely Sandstone (figs. 5, 6), a compositionally and texturally mature quartz-sand grainflow deposit reworked from the shelf (Viele and
Thomas 1989). Detrital zircons from the Blakely
Sandstone have dates that cluster at 1003–1188 and
2679–2722 Ma as well as scattered dates of 1271,
1334, and 1744 Ma (fig. 4; Gleason et al. 2002). The
dates of 1271–1334 Ma are similar to dates of 1284–
1407 Ma for granite boulders in the Blakely Sandstone (Bowring 1984), suggesting that both boulderand sand-sized detritus were supplied to the
off-shelf slope from the Granite-Rhyolite province
at the continental margin (figs. 5, 6).
The transgressive passive-margin deposits overlapped the rifted margin and progressively onlapped Grenville-age basement rocks (Read in Rankin
et al. 1989), consistent with the abundance of
Grenville-age detrital zircons in the basal sandstone. The entire rim of Grenville rocks adjacent
to the Laurentian margin may have been covered
by sediment and protected from erosion before deposition of the sandstones that contain Grenvilleage zircons within both the shelf and off-shelf
passive-margin successions (fig. 6), implying some
reworking of Grenville detritus (Gleason et al.
2002). Transcontinental drainages distributed sediment from the Grenville orogen across the Laurentian continent in the late Proterozoic (Rainbird
et al. 1997), suggesting a source for recycled Gren-
Journal of Geology
ALLEGHANIAN OROGENIC DETRITAL EVIDENCE
ville detritus from intracratonic Proterozoic sedimentary basins. The older (pre-Grenville) zircons
in the passive-margin sandstones may represent direct sediment transport from primary sources in the
midcontinent orogens and Superior province (e.g.,
Cawood and Nemchin 2001).
Taconic Orogen.
Plutons in the Appalachian
Piedmont (∼440–490 Ma) are interpreted to be the
eroded roots of Ordovician volcanic systems of the
Taconic orogeny (Shaw and Wasserburg 1984;
Tucker and Robinson 1990; Sevigny and Hanson
1993; Sinha et al. 1997; Karabinos et al. 1998; Coler
et al. 2000; McClellan and Miller 2000; Miller et
al. 2000; Aleinikoff et al. 2002). Widespread Kbentonite beds in the foreland stratigraphy document Taconic volcanism (e.g., Kay 1937, 1943; Kolata et al. 1996, 1998). Zircons with U-Pb dates of
453.1 Ⳳ 1.3 and 454.5 Ⳳ 0.5 Ma from the Kbentonites confirm syndepositional volcanism during the Late Ordovician (Tucker and McKerrow
1995). Metamorphism temporally and spatially
overlaps Taconic magmatism (Sutter et al. 1985;
Bosbyshell et al. 1998; Ratcliffe et al. 1998; Miller
et al. 2000).
Synorogenic clastic-wedge deposits of Middle Ordovician to Silurian age document transport of detritus from the Taconic orogen into the foreland
basin (e.g., Thomas 1977; Drake et al. 1989). Detrital zircons in the Taconic synorogenic sediment
are dominantly of Grenville age (fig. 4; Gray and
Zeitler 1997; Cawood and Nemchin 2001; McLennan et al. 2001). Ordovician clastic-wedge sandstones in Newfoundland also have clusters of zircon ages at ∼1800 and 2700–2800 Ma (Cawood and
Nemchin 2001). Although Taconic clastic-wedge
deposits bracket the ages of volcanic, plutonic, and
metamorphic rocks, they contain no detrital zircons from the contemporaneous igneous or metamorphic rocks or from the Taconic-age volcanoes
that are documented by numerous bentonite beds
in the foreland stratigraphy.
In the Ouachita succession of off-shelf passivemargin strata, compositional immaturity of the laterally discontinuous Silurian Blaylock Sandstone
(fig. 5) suggests a synorogenic provenance (Lowe
1989; Viele and Thomas 1989). Ages of detrital zircons from the Blaylock Sandstone cluster at 980–
1186 and 1321–1409 Ma and include a single grain
at 486 Ⳳ 51 Ma (fig. 4; Gleason et al. 2002). The
single young zircon suggests that Taconic plutons
might have been unroofed to supply detritus to the
continental slope. Alternatively, the large age uncertainty spans the age of synrift igneous rocks
along the Southern Oklahoma fault system (fig. 5;
Hogan and Gilbert 1998; Thomas et al. 2000).
31
Acadian Orogen. Plutons contemporaneous with
the Acadian orogeny (∼350–420 Ma) are distributed
along the Appalachian Piedmont but are concentrated in the northern Appalachians (Osberg et al.
1989; Eusden et al. 2000; Miller et al. 2000). Widespread bentonite beds within the Appalachian foreland contain zircons, the U-Pb ages of which confirm syndepositional volcanism during Devonian
orogenic events (Tucker et al. 1998). Regional metamorphism is contemporaneous with Acadian magmatism (Osberg et al. 1989; Hames et al. 1991; Eusden et al. 2000).
Synorogenic clastic-wedge deposits of Devonian
to Early Mississippian age document the Acadian
orogeny in the foreland (e.g., Thomas 1977; Osberg
et al. 1989). Detrital zircons from the Devonian
Walton Formation of the Catskill Group (Acadian
synorogenic clastic wedge) in southern New York
have a restricted distribution of ages, including
clusters of ages at 1018–1258 and 419–467 Ma (fig.
4; McLennan et al. 2001). The Grenville-age zircons
document sedimentary detritus from thrust sheets
of Grenville basement, Grenville-derived sediment
(such as synrift and passive-margin deposits that
were excluded from the craton-derived dispersal
systems of older detritus), or Taconic clastic-wedge
rocks that contain Grenville-age detrital zircons.
The younger zircons represent rocks that crystallized during the Taconic orogeny and possibly the
earliest part of the Acadian orogeny.
Accreted Terranes. Several of the exotic terranes
that were accreted to the Laurentian margin during
the Paleozoic orogenies (fig. 1; Secor et al. 1986;
Dallmeyer 1988; Horton et al. 1989; Getty and Gromet 1992) have components with crystallization
ages that contrast with the geologic history of Laurentia (e.g., Mueller et al. 1994; Samson 2001).The
terranes include zircons of distinctive ages (e.g.,
∼2200-Ma Gondwana basement to PennsylvanianPermian granites in the Suwannee terrane [Heatherington et al. 1999]; 550–630 Ma in the Carolina
terrane [Wortman et al. 2000; Hibbard et al. 2002];
and 530–630 Ma in the Avalon terrane [Zartman et
al. 1988]). Timing of accretion of the various terranes is poorly constrained, but estimates range
from Taconic (Ordovician-Silurian accretion of the
Carolina terrane [Hibbard 2000]) to Alleghanian
(Pennsylvanian-Permian accretion of the African
Suwannee terrane [Thomas et al. 1989]). Despite
the range of possible times of accretion and distinctive ages of zircons, none of the accreted terranes presently exposed in the Piedmont has been
identified as a source of detrital zircons in the Taconic or Acadian clastic wedges (fig. 4).
Alleghanian Orogen.
Ages of Alleghanian plu-
32
W. A . T H O M A S E T A L .
tons (fig. 1) are 300–330 Ma in the southern Appalachian Piedmont (Sinha and Zietz 1982; Samson
et al. 1995; Coler et al. 2000; Samson 2001) and
270–330 Ma in New England (Aleinikoff et al. 1985;
Zartman and Hermes 1987; Tomascak et al. 1996).
Corresponding to the magmatism, several tonsteins (volcanic ash beds) are identified within
Pennsylvanian-age coal beds (Lawrence, Lower Kittanning, Fire Clay, and Upper Banner) in the Appalachian basin (Congdon et al. 1992). The Upper
Banner and Fire Clay tonsteins have been radiometrically dated at 316 Ⳳ 1 Ma (U-Pb zircon) and
311.2 Ⳳ 0.7 Ma (40Ar/39Ar), respectively (Lyons et al.
1992, 1997; Kunk and Rice 1994), indicating that
deposition was contemporaneous with volcanism
in the southern Appalachians. Widespread metamorphism accompanied the magmatism (e.g., Reck
and Mosher 1988; Getty and Gromet 1992; Goldberg and Dallmeyer 1997; Wortman et al. 1998; Krol
et al. 1999).
The detrital zircons from the Pennsylvanian-age
Sewanee Conglomerate and Cross Mountain Formation within the Alleghanian foreland basin in
Tennessee represent a wide range of crystallization ages, but zircon crystals contemporaneous
with the Alleghanian orogeny are lacking (figs. 2–
4; tables 1, 2). The ages of the youngest zircons
overlap with the ages of Taconic and Acadian plutonic rocks, indicating that these orogenic cores
had been incorporated into the Alleghanian orogen,
unroofed, and integrated into the drainage network.
Grenville-age detrital zircons indicate primary
Grenville basement sources and/or recycling of
Grenville zircons from a variety of rocks incorporated in the Alleghanian orogen, including Taconic
and Acadian synorogenic clastic wedges, passivemargin strata, and synrift deposits. Sources of preGrenville zircons are more problematic, because
the older zircons might have been dispersed from
primary sources in the Laurentian craton through
a long longitudinal river system to the foreland basin in Tennessee. Consistent with an orogenic
source for the younger zircons, however, all of the
pre-Grenville zircons may have been recycled from
craton-derived synrift or passive-margin sediment
or from Taconic or Acadian synorogenic deposits,
which record earlier episodes of recycling of cratonderived synrift sediment.
Zircons from quartz pebbles in the Middle Pennsylvanian Sharp Mountain Member of the Pottsville Formation in the Appalachian foreland basin
in Pennsylvania also have been dated by U-Pb techniques (figs. 1, 2, 4; Gray and Zeitler 1997). The
ages of zircons include two clusters, 400–450 and
950–1200 Ma, and are scattered at 450–650, 1200–
1900, and 2080 Ma. These dates are similar to those
obtained from the Pennsylvanian-age sandstones in
Tennessee in representing Acadian, Taconic, and
Grenville ages. The lack of Superior-age zircons
(2600–2800 Ma) in the Sharp Mountain Member
(Gray and Zeitler 1997) negates sediment dispersal
from the shield. A detrital zircon age of 2080 Ma
suggests derivation from a Gondwanan craton
(Gray and Zeitler 1997), and zircon ages of 580–640
Ma are consistent with the peri-Gondwanan Avalon terrane (Zartman et al. 1988), suggesting detritus from accreted terranes.
Paleocurrent measurements on trough cross-beds
in Pennsylvania indicate shifts from northwestward drainage during deposition of the Tumbling
Run and Schuylkill Members of the Pottsville Formation to southwestward drainage during deposition of the Sharp Mountain Member and back to
northwestward drainage during deposition of the
Llewellyn Formation (figs. 1, 2; Meckel 1967; Robinson and Prave 1995). The northwestward drainages are assumed to transect the orogenic belt,
whereas the southwestward drainage represents
longitudinal (orogen-parallel) sediment transport.
Paleocurrents of the Sharon-Olean conglomerates
in northwestern Pennsylvania and northeastern
Ohio indicate southward drainage (figs. 1, 2;
Meckel 1967); however, that drainage was separated by a paleotopographic barrier from the coeval
northwestward drainage during Tumbling RunSchuylkill deposition in eastern Pennsylvania (Edmunds et al. 1999). The potential provenance for
southwestward, longitudinal drainage of the Sharp
Mountain Member was limited by the late Paleozoic tectonic framework of the northern Appalachians, where a system of strike-slip faults defined
the Maritimes basin (e.g., Bradley 1982; Hyde 1995)
and interior drainage supplied sediment to the basin from latest Devonian into Permian time (fig. 1;
Fralick and Schenk 1981; McCabe and Schenk
1982; Thomas and Schenk 1988). The northern Appalachian tributary headwaters of southwestward
drainage were restricted by a drainage divide around
the Maritimes basin (fig. 1), consistent with recycling of sediment from pre-Pennsylvanian rocks in
Appalachian thrust sheets (Gray and Zeitler 1997).
Summary and Conclusions
Pennsylvanian-age sandstones within the Alleghanian synorogenic clastic wedge in the Tennessee
salient of the Appalachian thrust belt contain detrital zircons with ages of 365–430, 435–470, 770,
900–1150, 1150–1380, 1450–1540, 1550–1820, 2120,
and 2685–2860 Ma; however, the detrital-zircon
Journal of Geology
ALLEGHANIAN OROGENIC DETRITAL EVIDENCE
population includes no zircons that crystallized
during the Alleghanian orogeny. Active Alleghanian volcanoes (documented by tonsteins) did not
contribute significant fluvial sediment to the foreland basin. The ages of the detrital-zircon population are compatible with an orogenic provenance
that included thrust sheets of remobilized Acadian
and Taconic crystalline rocks, passive-margin and
synrift sedimentary rocks containing Grenville and
older craton-derived detrital zircons, synrift igneous rocks, possibly Grenville basement, and possibly non-Laurentian accreted terranes (fig. 7).
The zircon population of Pennsylvanian-age conglomerate (Gray and Zeitler 1997) in the Pennsylvania salient of the Alleghanian thrust belt is similar to that of the sandstones in Tennessee with
some important differences (fig. 4; tables 1, 2). The
dominance of Grenville and Acadian-Taconic zircons in Pennsylvania indicates a provenance in the
Appalachian orogenic belt, compatible with drainage from the northern Appalachians. The lack of
Superior-age detrital zircons in Pennsylvania may
be explained by local variations in craton-derived
components in synrift sediment as documented in
Newfoundland. Detrital zircon ages of 580–640 Ma
and 2080 Ma suggest possible incorporation of
Gondwanan accreted terranes into the drainage
system.
A first-cycle provenance in various age provinces
of the Laurentian craton cannot be eliminated completely for the older detrital zircons in Pennsylvanian-age sandstones. Nevertheless, the detrital zircon age populations of synrift, passive-margin, and
33
Taconic and Acadian synorogenic rocks span the
entire age range of the components of the Laurentian craton, indicating that all of the older (originally craton-derived) zircons could have been recycled from an orogenic provenance into the
Alleghanian sandstones. Within the Appalachian
succession, the progressive addition of Paleozoic
(Taconic and Acadian) zircons into the foreland basin clearly precludes exclusive derivation from the
Archean- to Proterozoic-age Laurentian craton. A
shift in the ␧Nd isotopic composition beginning in
the Middle Ordovician (Taconic) and persisting
throughout the Paleozoic also suggests a change
from passive-margin drainages headed in the Laurentian shield to craton-directed drainages headed
in the orogenic belt and dominated by Grenvilleage basement (Gleason et al. 1994, 1995, 2002; Andersen and Samson 1995; Bock et al. 1998). The lack
of contemporaneous zircons indicates that drainage
to the foreland basin was not integrated through
the orogenic hinterland and/or that Alleghanian
plutonic and metamorphic rocks were not rapidly
exhumed.
The detrital zircon populations of Appalachian
clastic wedges indicate a provenance dominated by
basement and recycled cover strata of the Laurentian margin. Tectonic models for Appalachian
foreland-basin subsidence and subsequent filling
should consider the proximal gravitational load to
be imbricated rocks of the Laurentian margin (e.g.,
Jordan 1981). In models of progressive synorogenic
denudation of a mountain range (e.g., Willett 1999),
detrital zircons should be useful to define the ex-
Figure 7. Schematic conceptual cross section (not to scale) to illustrate the possible sources of detrital zircons for
deposition in the Alleghanian foreland basin. Of particular importance, old craton-derived zircons in synrift and
passive-margin sediments have been reworked into Taconic and Acadian synorogenic deposits, and all of these rocks
are potential proximal sources for Alleghanian detritus.
34
W. A . T H O M A S E T A L .
tent of the headward advance of transverse drainages. The ages of detrital zircons within the Appalachian synorogenic clastic wedges indicate that
the preorogenic architecture of the plate margin exerts a primary control on the construction and erosion of a mountain belt.
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