ABSTRACT
Carboniferous strata of the famous Joggins fossil cliffs hold a
unique place in the history of geology. Made famous by the fossil
discoveries of Lyell and Dawson in the mid 1800s, the cliffs continue to
yield important information about paleobiology. The Joggins Formation
(of probable Langsettian age) has been completely remeasured for the
first time since Logan and Dawson's pioneering studies, and a
visual log and a map of the foreshore illustrate the 915.5 m of strata
along Chignecto Bay. Formation boundaries are formally described, and
two informal members are abandoned. The formation is divided into 14
cycles, most of which commence with major transgressions represented by
the open-water facies assemblage, some faunal elements of which show a
restricted-marine affinity. Higher in the cycles, the re-advance of
coastal and alluvial systems yielded poorly and well drained facies
assemblages, respectively. The main levels of standing trees, dominated
by lycopsids, were entombed where distributary channels brought sand
into coastal wetlands. Some trees contain tetrapods and invertebrates,
which may have sought refuge or become trapped in hollow trees.
Cordaitalean (gymnosperm) forests covered the alluvial plains and
basin-margin uplands, and were periodically swept by wildfires. The
predominance of flooding surfaces and the apparent absence of lowstand
exposure surfaces reflect the rapid subsidence of the Cumberland Basin
controlled by active basin-margin faults and salt withdrawal. The cycles
may reflect tectonic events, glacioeustatic sea-level fluctuations,
and/or variations in sediment flux.
RESUME
Les strates carboniferes des celebres falaises fossiliferes de
Joggins occupent une place unique au sein de l'histoire de la
geologie. Devenues celebres a la suite des decouvertes de fossiles de
Lyell et Dawson vers le milieu du 19e siecle, les falaises continuent a
fournir des donnees precieuses au sujet de la paleobiologie. La
Formation de Joggins (qui remonte vraisemblablement au Langsettien) a
ete entierement remesuree pour la premiere fois depuis les premieres
etudes importantes du secteur realisees par Logan et Dawson; une
description visuelle et une carte de l'estran illustrent les 915,5
metres de strates le long de la baie Chignectou. L'etude decrit
officiellement les limites de la formation et abandonne deux membres
officieux. La formation est subdivisee en 14 cycles dont la majorite
commencent avec des transgressions importantes representees par
l'assemblage de facies en eaux libres, dont certains elements
fauniques presentent une affinite marine restreinte. a des niveaux
superieurs des cycles, la recurrence des systemes cotiers et alluviaux
fournit des assemblages de facies mal draines et bien draines,
respectivement. Les principaux niveaux d'arbres sur pieds, a
predominance de lycopsides, ont ete enfouis dans des secteurs ou des
canaux tertiaires ont apporte du sable a l'interieur des terres
humides cotieres. Certains arbres renferment des tetrapodes et des
invertebres, lesquels pourraient avoir cherche refuge ou s'etre
retrouves prisonniers dans des arbres creux. Des forets cordaitaleennes
(gymnospermes) ont couvert les plaines alluviales et les terres hautes
de marge de bassin, et ont periodiquement ete balayees par des incendies
de foret. La predominance de surfaces d'inondation et
l'absence apparente de surfaces d'affleurement de bas niveau
temoignent de la subsidence rapide du bassin de Cumberland, controlee
par des failles de marge de bassin actives et un retrait du sel. Les
cycles pourraient correspondre a des evenements tectoniques, a des
fluctuations glacio-eustatiques du niveau de la mer ou a des variations
du debit de sediments.
[Traduit par la redaction]
THE JOGGINS CLIFFS
The Joggins fossil cliffs constitute one of the world's most
remarkable and historic stratigraphic sections. The cliffs extend from
Lower Cove past Joggins village to McCarron's Creek, bordering a
broad tidal platform along Chignecto Bay, where some of the world's
highest tides continually erode Carboniferous strata of the Cumberland
Basin (Figs. 1, 2). The Joggins Formation comprises only 2.8 km of a 30
km long continuous coastal section that Sir Charles Lyell (1871, p. 410)
described as "the finest example in the world of a natural
[Carboniferous coal measures] exposure", a view that is still
widely accepted.
[FIGURES 1-2 OMITTED]
Early accounts by Jackson and Alger (1828), Brown and Smith (1829),
and Gesner (1836) brought the Joggins section to the attention of the
world. Lyell visited Joggins during his first visit to North America and
was deeply impressed by the spectacular geological features (Lyell 1845;
Scott 1998). In the following year, Joggins hosted William Logan, the
head of the newly constituted Geological Survey of Canada, who recorded
a 14 570 foot (4441 m), virtually continuous section along Chignecto Bay
(Logan 1845; Rygel and Shipley 2005).
For the remainder of the century, Lyell, J.W. Dawson and other
Victorian luminaries repeatedly visited the cliffs, a particular
highlight being the discovery in 1852 of tetrapod bones and land snails
within tree trunks (Lyell and Dawson 1853; Dawson 1878, 1882). In many
ways, Joggins was to Lyell what the Galapagos Islands were to Darwin
(Calder 2003). Lyell's research at Joggins, following the earlier
success of "Principles of Geology" (Lyell 1830-1833), was
pivotal in establishing the stratigraphic record as an archive of
Earth's evolving landscape. Joggins is mentioned several times in
Charles Darwin's "Origin of Species" (1859). The cliffs
have yielded some of the world's best preserved fossil forests, the
earliest known true reptile (Hylonomus lyelli), and the first land snail
(Dendropupa vetusta--referred to as "miserable little
dendropupa" by Bishop Sam Wilberforce during the famous British
debate on evolution in the 1860s (Wilberforce 1860, p. 244).
Over the past 25 years, the natural laboratory of the Joggins
cliffs--renewed constantly by the rides--has experienced a surge of
scientific interest. Paleontological research has touched upon the main
forested levels and the entombment of standing trees and their contained
fossils (Rygel et al. 2004; Calder et al. in press), wetland and dryland
plant assemblages (Falcon-Lang and Scott 2000; Falcon-Lang 2003a,b),
charcoal and the wildfire record (Falcon-Lang 1999), and age assessment
(Dolby 1991; Utting and Wagner 2005). There have been discoveries of
reptiles, bivalves, ostracodes, gastropods and foraminifera, as well as
tetrapod trackways and other trace fossils (Solem and Yochelson 1979;
Archer et al. 1995; Reisz 1997; Hebert and Calder 2004; Falcon-Lang et
al. 2004a; Tibert and Dewey 2005). Sedimentological research has
investigated channel bodies (Rygel et al. 2001; Ryge1 2005), paleosols
(Smith 1991), sequence stratigraphy (Davies and Gibling 2003), and
composition of coals and carbonaceous strata (Gibling and Kalkreuth
1991; Hower et al. 2000). Scott (2001) and Falcon-Lang et al. (2004b)
presented general scientific accounts. In 2004, Joggins was proposed for
Canada's list of World Heritage nominations (Falcon-Lang and Calder
2004).
Despite the importance of the Joggins cliffs, the only formal
stratigraphic log has remained the detailed written descriptions of
Logan (1845) and Dawson (1854). Later workers have remeasured short
segments, and Ryan and Boehner (1994) recast Logan's log as a
simplified graphic log. We have remeasured the cliff and foreshore
section (915.5 m thick) from Lower Cove to south of Bell's Brook,
the lower 600 m of which was described by Davies and Gibling (2003), and
an interval of 145 m higher in the section by Teniere (1998). In
Appendix A we present a complete sedimentological log, with a notation
of Logan's numbered coals and Dawson's Coal Divisions, and
present a map of the foreshore that shows the stratigraphic height in
metres above the base of the Joggins Formation, so that historic and
future fossil discoveries may be accurately positioned. We also present
formal revision to the upper boundary of the Joggins Formation, and
review the formation's cyclicity and depositional setting. A
companion paper (Calder et al. 2005) presents the section at Lower Cove
and formally designates the latter strata as the Little River Formation,
and in so doing redefines the Joggins Formation. We hope that these
publications will encourage further research on this superb and historic
section.
STRATIGRAPHIC FRAMEWORK FOR THE JOGGINS FORMATION
Previous research
Ryan et al. (1991), Ryan and Boehner (1994) and Calder (1998)
summarized the history of stratigraphic nomenclature for the Cumberland
Basin, and Calder et al. (2005) set out more fully the history of
stratigraphic work in the Joggins area. Previous approaches to
subdividing the Joggins stratal interval are outlined in Figs. 3 and 4.
Despite its magnificence, the coastal exposure provides only a
two-dimensional view of the basinal strata, and a full regional
understanding is complicated by minimal exposure inland, the presence of
Chignecto Bay, and complex facies relationships southward towards the
Cobequid Highlands (Fig. 1), which was an active, fault-bounded upland
during deposition of the Joggins Formation. In proposing a stratigraphic
framework, every researcher since Logan has wrestled with this difficult
background.
[FIGURES 3-4 OMITTED]
Logan (1845) divided the Chignecto Bay strata into eight divisions
(numbered from the top downwards), placing the predominantly grey,
coal-bearing strata of the Joggins cliffs in Division 4. He measured
this interval as 2539' 1" (774 m) of grey and red mudstone,
sandstone, limestone, coal, carbonaceous shale and minor conglomerate.
Logan recognized 45 coal groups (identified with numbers) in Division 4,
and recorded an aggregate thickness of 37' 9.5" (11.5 m) of
coal and 23' 3" (7.1m) of limestone in the division. Red,
red-green and chocolate shale and sandstone constituted 264 m (34%) of
his section, the remainder being grey. In contrast, he noted the absence
of coal in the underlying Division 5, which he described as red mudstone
and grey and red sandstone, with a few green shale and calcareous
nodular horizons. This division, only partially exposed at Lower Cove,
was measured at 2082'(635 m) down to the top of "South
Reef", a prominent and thick sandstone body within Division 6.
Logan's Coal Group 45 lies slightly above the base of Division 4 at
Lower Cove, and was described as comprising 10' 2" (3.1 m) of
coal, carbonaceous shale and interbedded shale and sandstone, with a
basal thin coal 3" (7.5 cm) thick. These strata are underlain by
5' 6" (1.7 m) of underclay and red shale and sandstone, the
base of which marks the base of Division 4. The top of Coal Group 1
marks the top of Division 4 near Joggins village, where a 4' (1.2
m) limestone rests upon a 1' (0.3 m) coal. The overlying Division 3
has a lesser proportion of coal (22 seams with only 5' 5" or
1.7 m aggregate thickness), a greater proportion of red beds and, most
significantly, no limestone. Division 2 is mainly red sandstone and
mudstone.
Subsequent workers used aspects of Logan's classification.
Dawson (1854, 1855). divided Division 4 into 27 coal divisions
(designated with Roman numerals), and provided more detail for some of
Logan's coal groups. Dawson (1868) adopted aspects of British
terminology in identifying a Millstone Grit Series and a Middle Coal
Formation, a system that was modified by Bell (1912). Bell (1914)
subsequently proposed the name "Joggins Formation" for
Divisions 5, 4, 3 and part of 2, an interval 6886' (2099 m) thick,
and the name "Shulie Formation" for overlying strata (most of
Division 2 and Division 1). Bell discussed the rationale for combining
the Division 5 red beds and the Division 4 grey, coal-bearing beds into
one formation, inferring that a regional disconformity underlies
Division 5 within the Cumberland Basin (although it is not apparent in
the coastal section). He subsequently retracted the formation names
(Bell 1943) due to problems in correlating units inland, arising in part
from miscorrelation of the coal-bearing strata of Spicers Cove with
those of Division 4 to the north at Joggins. Bell combined the Joggins
and Shulie formations as the "Joggins member" of an undivided
Cumberland Group. Shaw (1951a,b) mapped coarser and finer rock units
within the basin, and assigned the Joggins grey strata to his unit 10,
noting that conglomeratic wedges near the Cobequid Highlands thin
northward and complicate the basinal stratigraphy. Copeland (1959)
followed this approach, and confirmed that coals thin inland within the
Joggins-Chignecto coalfield. Belt (1964) referred strata at this level
across the region to the "Coarse Clastic Facies", but Kelley
(1967) retained the Cumberland Group for the lower part of this
interval.
Ryan et al. (1991) reinstated the Joggins Formation and presented a
formal description with the Joggins cliffs as the type section. Ryan et
al. (1990) mapped the formation inland, and included Divisions 5, 4 and
the basal strata of Division 3 within the reconstituted Joggins
Formation, for a total thickness of 1433 m. Both Ryan et al. (1991) and
Ryan and Boehner (1994) illustrated member boundaries on a column
derived from Logan's section (their figs. 5 and 2-13,
respectively), but did not elaborate on them in the text. The figures
show three informal members: the Little River Bridge member (Division
5), the Coal Mine Point member, and the Bells Brook member (Division 4
and basal strata of Division 3), but these members were not formally
defined or shown on the accompanying maps.
[FIGURES 5-13 OMITTED]
In the coastal section, Logan's Division 5/6 boundary was
taken to mark the base of the Joggins Formation, and its top was placed
at the base of a thick sandstone body that ushers in a more
sand-dominated interval at the base of their new Springhill Mines
Formation (Ryan et al. 1991; Ryan and Boehner 1994). This upper boundary
was selected by Ryan et al. (1991) who placed the basal 168'
2" (51 m) of Division 3 within the Joggins Formation. This contact
has proven difficult to map.
Modifications to Joggins Formation definition at the type section
Remeasurement of the section (Appendix A of this paper, and
Appendix A of Calder et al. 2005) has led Calder et al. (2005, their
Appendix C) to redefine the Joggins Formation from the boundaries set
out by Ryan et al. (1991) (Fig. 4). Below, we describe the boundaries of
the revised formation at the coastal type section as they relate to our
recent sedimentological investigations.
Formation base and top at the type section
In the absence of a detailed measured section, Ryan et al. (1991)
included Division 5 within the Joggins Formation based on unconfirmed
reports of coals within the red beds. With the advantage of a detailed
measured section, Calder et al. (2005) reaffirm Logan's observation
that Division 5 lacks coal, fossiliferous limestone and grey mudstone.
They formally redefine Division 5 as the Little River Formation (635.8 m
thick) and, in so doing, also redefine the Joggins Formation. The base
of the Joggins Formation as reformulated by Calder et al. (2005)
coincides with the lowermost coal exposed in the section, within Coal
Group 45 and 1.7 m above the base of Division 4 as originally defined by
Logan (1845). This approach essentially retains Logan's Division 5
as a unit distinct from both the underlying Boss Point Formation and the
overlying Joggins Formation and, in consequence clarifies the definition
of the Joggins Formation as a coal-bearing unit with co-occurring
bivalve-bearing limestones.
The formation top is redesignated at the top of the uppermost
limestone unit within coal group 1 south of Bells Brook in keeping with
the original definition of Division 4 by Logan (1845), as opposed to the
channel-sandstone base at the higher level designated by Ryan et al.
(1991). This limestone/coal interval is unusually thick, and the
limestone is the topmost major calcareous unit in the type (cliff)
section. Mining records and mapping (Goudge 1945; Shaw 1951b; Copeland
1959; Ryan et al. 1990) show that limestone beds within the interval of
mined coal seams of the Joggins Formation (Coals 7-32) extend up to 40
km inland and can be interpreted as basin-wide flooding events (Calder
1994). Falcon-Lang (2003a) noted that these limestones and overlying
platy shales yield abundant remains of upland floral elements,
confirming that major flooding events inundated most of the basin. In
contrast, individual channel bodies have yet to be mapped inland, and
our observations in the coastal section (Davies and Gibling 2003; Rygel
2005) indicate that most channel-sandstone bodies are lensoidal and
discontinuous. Although thick intervals of coarser and finer strata tend
to be mappable within the basin at the formation scale (Shaw 1951b), the
lensoidal nature of and relative similarity between channel bodies in
the Joggins and Springhill Mines formations makes the uppermost
limestone a more useful lithostratigraphic surface.
The Joggins Formation as redefined has a total thickness of 915.5
m. Logan (1845) recorded this interval as 2533'7" (772 m), a
discrepancy likely resulting from the speed at which he measured the
section (Rygel and Shipley 2005).
Members
The Coal Mine Point and Bells Brook members of the Joggins
Formation were not formally defined by Ryan and Boehner (1994) and are
abandoned here. The boundary between them was designated as the top of a
sandstone just south of Bells Brook, but this bed is not prominent and
there is no indication that it is mappable. Between Lower Cove and Coal
Mine Point, Duff and Walton (1973) designated informal lower, middle and
upper beds, and Davies and Gibling (2003) and the present authors
recognize informal cycles 16 to 212 m thick, the bases of which are
located at prominent flooding levels marked by limestone, coal and grey
platy shale with siderite nodules (Appendix A). Some coal seams at cycle
bases are mappable inland (Goudge 1945; Copeland 1959) and some cycles
or groups of cycles could in the future merit member status if they
prove to be extensive.
AGE OF THE JOGGINS FORMATION
The formation has been dated on the basis of palynology as late
Langsettian (Dolby 1991). Recent investigation and taxonomic revision
ofthe macroflora suggest that the strata are most probably early
Langsettian (Utting and Wagner 2005), and the proximity of the
Namurian-Westphalian boundary is suggested by the presence of late
Namurian floral elements.
THE STRATIGRAPHIC SECTION
The stratigraphic section (Fig. 5; Appendix A) was measured
bed-by-bed with centimetre-scale resolution. Although measured at the
base of the accessible cliff, the section represents the exposed cliff
face more broadly and records lateral changes in thickness of channel
bodies and crevasse splays evident at the time of measurement. Figure 6
shows the stratigraphic level in the section (in metres above the base
of the Joggins Formation) for distinctive beds that protrude from the
adjacent tidal platform as resistant bodies, long known as
"reefs". Fine-grained beds, limestones and sharp-based
sandstones are generally continuous across the exposure area, but the
humerous lensoidal channel bodies exposed only in the foreshore are not
represented in the measured section. Considerable difficulty was
encountered in measuring an accurate section through the former mining
areas on the Joggins Seam due to waste heaps and the removal of the coal
seams. We record present exposures in this interval (~800 m to 870 m),
leaving Logan's section--which was measured prior to major coal
extraction--as a more complete representation. Red and drab intervals
are recorded to the left of the column; these colour designations are
highly generalized, and drab intervals in particular show wide variation
from dark to light grey and green. Most channel sandstones are
grey-brown regardless of their association, and their colour is
typically shown on the log as similar to the strata above and below.
The section is divided into 14 cycles (Fig. 7), the bases of which
are marked by limestone, coal or fossiliferous shale. The cycles are
divided in turn into stratal intervals that belong to the open-water,
poorly drained floodplain, and well drained floodplain facies
assemblages (Davies and Gibling 2003), the main features of which are
summarized briefly below. In the cycles, the three assemblages typically
succeed each other upwards, although the open-water facies assemblage is
not represented in some cycles.
FACIES ASSEMBLAGES AND FOSSIL GROUPS
Open-water facies assemblage
This assemblage represents major flooding events, some of which
probably inundated most of the western Cumberland Basin. Especially good
examples in the lower part of the formation (cycles 2-4) are represented
by a thin coal overlain by a dark limestone up to 1 m thick, which is
overlain in turn by several metres of grey siltstone capped abruptly or
gradationally by sandstone. The limestones are well cemented and stand
out on the foreshore. They are locally termed "clam coals"
because of their dark colour and the abundance of bivalves with
accessory ostracodes, spirorbids, arthropods, disarticulated fish and
plant fragments.
The overlying siltstones are laminated and platy weathering, and
contain discoidal siderite nodules. The siltstones contain bivalves and
ostracodes (generally confined to discrete levels), as well as drifted
plant material. Agglutinated foraminifera were obtained from some
samples (Archer et al. 1995). Capping the siltstones are sharp-based,
sheet-like sandstones a few metres thick, which extend across the cliff
and foreshore and are characterized by planar bedding and a flaggy
appearance. In a few instances, they comprise overlapping mounds up to
100 m in apparent width. The sandstones contain unidirectional ripple
cross-lamination, local mud drapes, and lineated plane beds, with wave
ripples and rare hummocky cross-stratification indicating wave activity.
Trace fossils include delicate grazing and walking traces (Archer et al.
1995) and, less commonly, resting traces (for example of limulids). A
few channel bodies cut the planar sandstones, with which they are
evidently closely associated. The topmost coarser beds contain roots,
which mark the re-establishment ofsubaerial conditions after the initial
flooding event.
The assemblage represents the establishment across the basin of a
restricted-marine gulf, perhaps similar to the modern Baltic Sea in its
partially enclosed nature and variable but generally low salinity
(Grasshoff 1975). Open-marine faunal elements have not been observed,
but the presence of certain taxa of bivalves, ostracodes, foraminifera
and trace fossils suggest at least brackish conditions (BeU 1914; Duff
and Walton 1973; Archer et al. 1995; Skilliter 2001; Tibert and Dewey
2005). Strontium isotope data from fish material also suggest marine
influence (Calder 1998), although mineralogical and geochemical data
from some bivalve shells are consistent with a freshwater setting (Brand
1994). During some flooding events at cycle bases, the presence of a
basal coal suggests that peat formation initially kept pace with rising
water level. When the rate of sea-level rise exceeded that of peat
accumulation, the area was transformed into a shallow bay where
faunal-concentrate layers accumulated. The upward change to siltstone
indicates the renewed advance of the coastal plain as the rate of
sea-level rise decreased. Progradation culminated in shallow-water sands
that represent thin shorefaces and small delta lobes derived from the
associated channels. Wave activity was prominent, but sedimentological
evidence for tidal influence is restricted to mud drapes at a few levels
(Skilliter 2001).
Drifted lycopsid plants predominate in the basal limestones,
whereas overlying siltstones and sandstones contain a mixed suite of
drifted gymnosperms (cordaitaleans), sphenopsids (primarily
calamiteans), pteridosperms and putative progymnosperms (Falcon-Lang
2003a). The high proportion of progymnosperms and gymnosperms suggests
that the basin floor was almost entirely drowned, greatly reducing the
area of coastal swamps. Under these conditions, elements of the upland
vegetation (Falcon-Lang and Scott 2000) brought to the sea by rivers
were preferentially concentrated in the siltstones (an example of the
"Neves Effect" of Chaloner 1958). The presence ofstigmarian
roots within some limestone beds points to near emergent, shallow
conditions in some cases, and maximum water depth during deposition of
open-water facies was probably only a few metres to a few tens of
metres.
Poorly drained floodplain assemblage
Joggins is justifiably famous for its poorly drained floodplain
deposits, which contain the spectacular fossil forest horizons.
Sandstone and green/grey mudstone (commonly intensively rooted), are
accompanied by coal, carbonaceous shale and minor limestone, with
siderite nodules. Bivalves and ostracodes are generally less common than
in the open-water assemblage. The assemblage includes thin grey
carbonaceous shales that separate red beds near cycle tops from coal and
limestone at the base of the next cycle.
Especially prominent are sheet-like, heterolithic units of
sandstone and mudstone, several metres thick, which extend across the
cliffs and foreshore and contain many entombed erect trees. Splendid
examples include two forested intervals below the Fundy Seam (404-420 m;
Calder et al. in press), two intervals just below the Forty Brine Seam
(539-546 m) and the lower reef just north of Coal Mine Point (628-630
m). Charles Lyell was struck by the preserved height of the trees, which
he estimated to be up to 7.6 m tall (Lyell 1842, 1845). The maximum
height observed over the past three decades has been 6 m (Calder et al.
in press). Vegetation includes abundant lycopsids and sphenopsids in
situ (Falcon-Lang 1999; Calder et al. in press) with a compression
macrofloral record comprising cordaitalean gymnosperms, pteridosperms,
ferns and putative progymnosperms (Calder et al. in press). Many
standing trees are bordered by scour fills of sandstone up to 2 m thick
with centroclinal (inward dipping) cross-stratification, and suites of
large sandy mounds (vegetation shadows) are common (Rygel et al. 2004).
Within this assemblage, Lyell and Dawson made their remarkable
discovery of fossils within tree trunks, mainly from the lower reef
("Lesser Reef" of Dawson 1882) at Coal Mine Point. The
tree-stump fauna is highly diverse, including eleven tetrapod genera,
abundant coprolites, and a variety of invertebrates--land snails
(Dendropupa), millipedes, arthropod fragments, and calcareous shells of
the annelid Spirorbis (Carroll et al. 1972). Charcoal fragments are
abundant within and adjacent to some trunks, testifying to wildfires
that swept the forests (Falcon-Lang 1999). The organisms may have taken
refuge within hollow trees or may have become entombed accidentally, and
some fragments may have been swept in by floods (Lyell and Dawson 1853;
Carroll et al. 1972; Scott 2001).
The heterolithic units are closely associated with narrow channel
bodies up to 3 m thick. A few much larger channel bodies are also
present in the assemblage. One large channel body at 580 m has incised 9
m through rooted grey mudstone and contains two vertically stacked
storeys. The major sandstone "reef" at Coal Mine Point
(637-648 m) is a channel body that contains trough cross-beds,
scroll-bar forms and lateral accretion surfaces (Rygel 2005). This
channel body contains especially spectacular examples of Diplichnites
(large trackways attributed to the arthropod Arthropleura: Ferguson
1975).
Many of the coal groups (marked on the section in Appendix A) lie
within this assemblage, which includes the main economic seams of the
formerly worked Joggins-Chignecto Coalfield: most notably the Fundy
(coal 29a), Forty Brine (coal 20), Kimberly (coal 14), Queen (coal 8)
and Joggins (coal 7) seams. The bituminous coals are sulphur-rich
(Copeland 1959; Skilliter 2001) with prominent mudstone partings and
locally high concentrations of Zn, Pb and As (Hower et al. 2000).
The strata were deposited in wetlands akin to those of the modern
Mississippi Delta (Coleman and Prior 1980; Tye and Coleman 1989),
although the geomorphic form of the coastal system is not known. Small
distributary channels traversed the coastal plain and brought sand and
mud to the adjacent fresh to brackish bays during repeated flood events,
depositing characteristically heterolithic sediment as interdistributary
crevasse splays and bay fills. These sedimentation events entombed the
standing trees (Calder et al. in press) and created scour hollows and
vegetation shadows around the trunks (Rygel et al. 2004). At the level
of the upper Fundy forest (419 m level in Appendix A), thin sandstone
sheets can be traced from the margins of channel bodies on the tidal
platform into scour fills around standing trees exposed in the cliffs.
The forests were affected by wildfires that may have been instrumental
in hollowing out the trunks and providing shelter for early tetrapods
and other organisms. The channel body at the 580 m level is interpreted
as a large distributary channel, based on its incised nature and
aggradational style, whereas the Coal Mine Point channel body represents
a meandering river that advanced over bayfills, much as the modern
Atchafalaya River of Louisiana advanced rapidly once it had filled
Atchafalaya Bay (Tye and Coleman 1989). Thin poorly drained intervals at
cycle tops represent incipient drowning of the coastal zone prior to the
main transgression.
The coals represent planar (groundwater-fed) mires (Hower et al.
2000; Calder et al. in press) where, for prolonged periods, peat
accumulated away from detrital supply, although floods periodically
generated muddy partings. Several economic seams cap heterolithic units
or channel bodies, suggesting that the precursor peat accumulated in
freshwater settings following abandonment of a local distributary. These
thick coals may be the updip equivalent of marine flooding events, and
the high sulphur levels of many coals suggest that marine, sulphate-rich
waters influenced the peats, probably after the mires were drowned by
sea-level rise. Many drab mudstones are hydromorphic paleosols that
formed under variable redoxymorphic conditions, and red and red/grey
mottled intervals testify to episodes of soil formation under oxidizing
conditions (Smith 1991). The 595-612 m interval of cycle 9 contains
stratified red and grey mudstone without coal or invertebrate fossils,
suggesting that oxidized mud was washed into clastic-dominated lakes.
Well drained floodplain assemblage
This predominantly red bed assemblage comprises red mudstone and
sandstone, with minor grey mudstone, rare coal and ostracode-bearing
limestone. Although not highly fossiliferous, these strata have recently
yielded some unusual fossil diScoveries (Hebert and Calder 2004), as
outlined below. Cycle 4 contains an especially thick red bed interval.
Channel bodies are narrow and up to 6 m thick with an aggradational
style of filling, and in places several bodies lie at the same
stratigraphic level in the cliffs (Rygel 2005). Most are associated with
heterolithic sheets of sandstone and mudstone that typically thin away
from the channel bodies and represent levee and crevasse splay
complexes. The channels are filled with grey and red sandstone and
mudstone, with local conglomerates composed of reworked carbonate
(paleosol) fragments. Within cycles 1 and 3, some large channel bodies
contain smaller channel fills, suggesting that the bodies are small
dryland valleys. The red muds are poorly stratified and contain
scattered calcareous nodules, although petrocalcic horizons were not
observed.
Standing trees are restricted to poorly preserved stump casts, and
narrow hollow fills with abundant underlying roots marking the former
positions of trees, since decayed (Rygel et al. 2004). Charcoal and
other floral remains in channel bodies are dominated by cordaitaleans,
with minor pteridosperms, sphenopsids and lycopsids--the minor
constituents typically confined to channel-margin situations
(Falcon-Lang 1999, 2003b; Falcon-Lang and Scott 2000). One channel body
at 270-274 m, known informally as the "Hebert beds", contains
abundant charcoal as well as tetrapod material, shells up to 23 cm long
of the unionid bivalve Archanodon, and land snails (Dendropupa)
(Falcon-Lang et al. 2004a; Hebert and Calder 2004). Other channel bodies
have yielded large arthropod trackways (Diplichnites).
The assemblage represents the alluvial plain of a seasonal dryland
traversed by suites of narrow channels that probably had an anastomosing
planform (Rygel 2005), as indicated by multiple, narrow channels at
similar levels connected by sheet sandstones ("ribbon tiers"
of Kraus and Wells 1999). The setting may have resembled that of the
Channel Country of Australia with its dryland anastomosing systems and
water holes (Gibling et al 1998); Rust etal. (1984) also drew on the
Channel Country in interpreting the overlying Springhill Mines
Formation. The red floodplain muds are immature, cumulative paleosols
that formed under a humid seasonal climate (Smith 1991). The abundance
of cordaitalean charcoal in some dryland channels suggests that the
seasonally dry floodplains were covered with a fire-prone and
ecologically stressed assemblage dominated by gymnosperms (Falcon-Lang
2003b; Falcon-Lang et al. 2004a). Riparian (channel-margin) settings
permitted the local growth of vegetation more akin to the wetlands, and
wildfires were common, perhaps promoted by elevated levels of
atmospheric oxygen (Robinson 1991). The unusual biota preserved within
the "Hebert beds" suggest that the parent channels provided
intermittent water holes where organisms continued to flourish during
seasonal low-stage flow or more prolonged droughts (Falcon-Lang et al.
2004a).
FORMATION-SCALE TRENDS
Cyclic patterns
The 14 cycles recognized in the 915.5 m of the Joggins Formation
range in thickness from 16 to 212 m, averaging 65 m. Nine cycles are 16
to 52 m thick, three are 52 to 99 m thick, and the remaining two are 158
m and 212 m thick, respectively. Facies distribution was used to
construct a relative base-level curve for the formation (Davies and
Gibling 2003; Fig. 7). Unfortunately, the present lack of firm
biostratigraphic boundaries and absolute dates for the section precludes
the determination of cycle durations and accumulation rates.
Relatively straightforward facies patterns are evident in cycles 1
to 4 and in the basal part of cycle 5. These intervals show a systematic
upward succession from open-water facies, which mark major
transgressions, to poorly drained and well drained facies, marking
regressions. At some levels, thin occurrences of poorly drained facies
below open-water intervals herald the base of the next cycle, denoting
the onset of base-level rise. Limestones and platy siltstones are
prominent, coals are thin and mainly underlie limestones, and the
prominent sharp-based sandstones that cap open-water deposits contain
many trace fossils. Cycle 5 constitutes the most prolonged period of red
bed accumulation, with alternate periods of poorly and well drained
conditions and some thin coals in the upper 80 m.
The most marked and sustained lithological change within the
formation is the relatively abrupt change from well drained to poorly
drained floodplain deposits at the base of cycle 6 (Fig. 7), with a
suite of prominent coals; future mapping inland may provide
justification for identification of a member boundary at this level.
Cycles 6 to 8 are of moderate thickness, and usher in a period when the
area was dominated by coastal wetlands (represented by the poorly
drained assemblage), with only thin intervals of open-water deposits.
Coals are numerous and thick, and many of the most prominent fossil
forests are found in this interval (Calder et al. in press). Limestones
are generally scarce, apart from a thick bed at the base of cycle 8.
Cycle 9 commences with a well developed occurrence of open-water facies
above the Forty Brine Seam (Skilliter 2001), with limestones, mud
drapes, trace fossils (Archer et al. 1995), and a large distributary
channel body, passing upward into probable lacustrine deposits of
stratified red and grey beds.
Cycle 10 (158 m thick) marks the start of a 200 m interval of
alternate poorly drained and well drained deposits without open-water
sections and limestones (cycles 10-12). Prominent sets of fossiliferous
carbonaceous shales (cycle 10) or thick coals (Queen and Joggins seams,
cycles 11 and 12) mark cycle bases, but several thin coals delineate
minor transgressions within the numbered cycles. Limestones mark the
base of cycles 13 and 14 and the Joggins / Springhill Mines formation
contact.
Following the abrupt onset of wetland conditions at its base, the
Joggins Formation records a punctuated set of advances and retreats of
the coastal zone (Fig. 7). Along-term balance seems to have been
maintained between accommodation creation and sediment supply, such that
the study area remained close to the coastal zone for much of Joggins
Formation time, with periods of more sustained open-water, wetland or
dryland conditions. Thick limestones and thick coals tend to be mutually
exclusive (Fig. 7): thin coals underlie many limestones, but thick coals
rarely have limestone caps, the most notable exception being the Forty
Brine Seam (coal 20). This pattern probably reflects variations in the
magnitude and rate of base-level rise. Large base-level rises would tend
to flood much of the Cumberland Basin, resulting in reduced sediment
flux to open-water areas and the accumulation of fossil-concentrate
limestone. Rapid base-level rise would tend to outpace the rate of peat
accumulation, resulting in thin peats only. In contrast, thick peats
(coals) probably accumulated where modest or slow base-level rise caused
prolonged freshwater ponding inland of transgressive shorelines (Kosters
and Suter 1993). A rheotrophic (groundwater-influenced), planar
character is the hallmark of the coals of the Joggins Formation (Hower
et al. 2000; Calder et al. in press).
Sand accumulated preferentially in the poorly drained floodplain
assemblage, where coastal bays formed repositories for coarse detritus.
In these areas, sand deposition was strongly focused into sheets, scour
fills and vegetation shadows where forested landscapes slowed
overtopping flood waters (Rygel et al. 2004). In contrast, shoreface and
delta-lobe sands of the open-water assemblage are relatively thin, and
dryland alluvial plains of the well drained assemblage include thick mud
intervals, with sands restricted to small channels, levees and splays.
Sequence stratigraphy
Many Carboniferous cycles (or cyclothems) reflect sea-level
fluctuations in the order oftens of metres in amplitude caused by the
accumulation and melting of ice sheets in high southern latitudes
(Crowley and Baum 1991; Maynard and Leeder 1992; Soreghan and Giles
1999). Glacioeustasy in Carboniferous basins has commonly generated
stacked Exxon-type sequences with prominent sequence boundaries, valley
fills, maximum flooding surfaces and systems tracts (Hampson et al.
1999; Gibling et al. 2004). Such expressions of glacioeustasy may be
modified under conditions of unusually rapid subsidence, as at Joggins,
where the record of sea-level fall maybe suppressed and the record of
sea level rise may be strongly augmented, rendering the basin
susceptible to basin-wide flooding events marked by fauna-rich horizons.
At Joggins in the western Cumberland Basin, rapid subsidence reflects
the extensional basin setting, coupled with active withdrawal of Windsor
salt (Waldron and Rygel 2005). In consequence, the Joggins cycles
display what we consider to be a "tectonically controlled
architecture" characterized by multiple flooding surfaces (Davies
and Gibling 2003), including coal and fossiliferous limestone at cycle
bases that mark important episodes of sea-level rise. The overlying
strata lack clear evidence for sea-level fall such as profound valley
incision or well developed paleosols, although sharp-based shoreface and
delta-lobe sandstones may reflect in part modest falls of sea-level
(similar to those documented by Plint 1988). Large channel bodies appear
to represent distributary channels and meandering rivers within a
coastal plain setting, rather than recording profound basinward facies
shifts that could have emplaced proximal (braided or low-sinuosity)
river deposits over marine deposits. Small valley fills in cycles 2 and
3 appear to lie within red bed intervals, and need not imply major
basinward shifts of facies belts linked to base-level lowering. The
Joggins cycles may record glacial-interglacial transitions manifested in
an equatorial setting, periods of varied subsidence rate as faults moved
and salt migrated, variations in sediment flux, or combinations of all
three. However, regardless of the ultimate cause of the cycles, their
unusual architectural features are inferred to reflect extremely rapid
subsidence, in accord with observations in other high-subsidence
settings worldwide (Davies and Gibling 2003).
In the absence of sequence boundaries, the Joggins cycles can be
categorized as parasequence sets, in which the predominant coastal-plain
facies are composed of numerous thin parasequences and bounded by
flooding surfaces marked by limestones, coals and carbonaceous shales.
Thin drab intervals at cycle tops mark retrogradational parasequence
sets that culminated in profound flooding at the start of the overlying
cycle. As coastal rivers re-advanced, thick progradational parasequence
sets accumulated where tropical wetland deposits filled marine
embayments, until a dryland alluvial plain was established. Thereafter,
alluvial red beds accumulated, flooding surfaces become fewer and less
prominent, and trends of pro-, retro- or aggradation are difficult to
establish.
Because subsidence was so rapid, a remarkably complete record of
environments and the organisms that inhabited them is preserved in the
Joggins cycles. In particular, prolonged periods of wetland conditions,
during which sedimentation kept pace with subsidence, promoted the
repeated generation and burial of forests at many levels (Waldron and
Rygel 2005).
CONCLUSIONS
The Joggins Formation in the famous fossil cliffs along Chignecto
Bay, Nova Scotia, has been completely remeasured for the first time
since the mid 1800s. We present a visual log of 915.5 m of strata and
describe revised formation boundaries (formalized in Calder et al.
2005). The formation comprises stacked transgressive-regressive cycles
that typically commence with open-water facies with a restricted-marine
fauna, overlain by prograding coastal and alluvial deposits. The main
levels of standing trees, predominantly lycopsids, were entombed where
distributary channels brought sand into coastal wetlands, and some trees
contain tetrapod and invertebrate fossils. Fire-prone cordaitalean
(gymnosperm) forests covered the alluvial plains and basin-margin
uplands. The cycles may reflect tectonic or glacioeustatic events, or
variations in sediment flux. within the cycles, the predominance of
flooding surfaces and the apparent absence of lowstand exposure surfaces
reflect rapid subsidence of the Cumberland Basin, controlled by active
basin-margin faults and salt withdrawal.
Joggins is an unusual geological locality, and the many scientists,
university and school groups, and enthusiastic members of the public who
visit the cliffs each year bear testimony to the enduring fascination of
this special site. We offer here an illustrated log and foreshore map of
the Joggins Formation, along with a brief summary of especially
interesting features of the strata, to encourage continued interest in
the rocks and fossils. Given the huge and frequently overwhelming amount
of rock on display, we hope that the log will serve as a simple guide to
visitors with some knowledge of geology, as well as allowing specialists
to mark precisely the locations of new fossil discoveries. As Lyell and
Dawson realized more than 150 years ago, Joggins is all about ancient
landscapes inhabited by remarkable plants and animals, for the fossils
are at their most compelling when we can imagine them in the
environments where they lived more than 300 million years ago.
ACKNOWLEDGMENTS
We are most grateful to Peter Giles and Sue Johnson for their
thoughtful comments on the manuscript, to Rob Fensome for editorial
suggestions, and to Howard Falcon-Lang, Brian Hebert, Andrew Henry,
Adrienne Rygel, Don Reid and Leonard Wilson for assistance and
discussion. M. Gibling acknowledges funding from the Natural Sciences
and Engineering Research Council of Canada (NSERC Grant Number 13354),
the Petroleum Research Fund of the American Chemical Society (Grant
36917-AC8), and a research grant from Imperial Oil (Grant IO182DAL). M.
Rygel's research was supported by a Killam Predoctoral Scholarship
at Dalhousie University and grants from the American Association of
Petroleum Geologists, the Geological Society of America (GSA), and the
GSA Coal Geology Division (Medlin Award).
APPENDIX A
Measured section of the Joggins Formation from Lower Cove to south
of Bell's Brook.
[ILLUSTRATION OMITTED]
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Editorial responsibility: Robert A. Fensome
S.J. DAVIES (1) *., M.R. GIBLING (2), M.C. RYGEL (2) ([dagger]),
J.H. CALDER (3), AND D.M. SKILLITER (4)
(1.) Department of Geology, University of Leicester, Leicester LE1
7RH, U.K.
(2.) Department of Earth Sciences, Dalhousie University, Halifax,
Nova Scotia B3H 3J5 Canada
(3.) Nova Scotia Department of Natural Resources, P.O. Box 698,
Halifax, Nova Scotia B3J 2T9 Canada
(4.) Nova Scotia Museum, 1747 Summer St., Halifax, Nova Scotia B3H
3A6 Canada(
* corresponding author
([dagger]) Present address: Department of Geoscience, 214 Bessy
Hall, University of Nebraska-Lincoln, Lincoln, Nebraska, 68588, USA
Date received: 14 January 2005 [paragraph] Date accepted 9 August
2005