Table Of ContentI I
EarlyTerrestrial Plant Environments:
An Example from the Emsian of Gaspe, Canada
C. L Hotton, F. M. Hueber, D. H. Griffing, and j. S. Bridge
The origin of embryophytes in the Ordovician few paleoenvironmental studies of specific plant
and their subsequent radiation in the Silurian assemblages have been conducted (Andrews et
and Devonian were profoundly important al. 1977; Gensel and Andrews 1984; Edwards
events in the history of life. Land plants amelio and Fanning 1985; Scheckler l 986b; Kasper et
rated an initially hostile terrestrial environment, al. 1988), of which the most detailed is that by
paving the way for colonization by other organ Schweitzer (1983). However, these studies are
isms. Rhizoid and root activity reduced substrate limited by absence of sedimentological detail.
instability, promoted chemical weathering, and Because the degree to which a fossil assemblage
increased nutrient availability; the establish resembles the parent plant communities depends
ment of a plant canopy dampened fluctuations in large part on sedimentological and tapho
in humidity and temperature, greatly aug nomic processes, understanding the sedimento
mented primary productivity, and opened up logical context of fossil assemblages and associ
niches for exploitation by other organisms ated facies is cmcial to the reconstruction of their
(Beerbower 1985). Increased incorporation of living environment. Hence, we have undertaken
co2
into organic carbon by land plants over the a detailed study of the sedimen tology of fossil land
course of the iluro-Devonian was apparently a plant assemblages in the Emsian age Cap-aux-Os
primary factor in precipitating a sharp drop in Member of the Battery Point Formation, exposed
atmospheric C0 (Berner 1993, 1994; Mora et along Gaspe Bay, Quebec, Canada. The diverse
2
al. 1996; ee chapter 10), which in turn was likely Gaspe flora has been studied for over a century,
responsible at least in part for the Permo beginning with the pioneering work of J. W.
Carboniferous glaciations. Even the Frasnian Dawson (1859, 1871). Many subsequent studies
Famennian marine mass extinctions have been have established it as one of the best-described
attributed to coastal marine eutrophication floras of Early Devonian age (Hopping 1956;
fueled by tracheophyte radiation and evolution Daber 1960; Krause! and Weyland 1961; Banks
of deeply penetrating root systems over the and Davis 1969; Banks etal. 1975; Hueber 1967,
course of the Devonian (Algeo et al. 1995; Algeo 1971, I 983b, 1992; Gensel 1976, 1979, 1984;
and Scheckler 1998). Granoff et al. 1976; Remy et al. 1993; Stein et al.
Despite the significance of early land 1993). However, no paleoecological study of this
plant paleoecology, detailed studies of pre flora has been attempted.
Carboniferous plant assemblages are rare. A Although the fossil record suggests that by
179
180 Early Terrestrial Plant Environments
the Emsian, embryophytes had occupied the paper: ( 1) to characterize types of environments
land for roughly 70 million years (Gray 1993; occupied by plants; (2) to search for consistent
Strother et al. 1996), terrestrial ecosystems at associations between plant r.axa and environ
that time are still considered primitive. Absence ments; (3) to characterize the degree to which
of tetrapods and the plesiomorphic nature of plant assemblages were in fact monodominant;
arthropod faunas at this time suggest that ( 4) to investigate the extent to which early land
plant-animal interactions would have been lim plants were capable of occupying high-stress
ited (Selden and Edwards 1990; DiMichele et al. environments, focusing specifically on brackish
1992). Plants are believed to have occupied sites water habitats. Finally, we document the taxo
in rare sexual reproductive events and then nomic and diversity disjunction between mega
formed monospecific stands through extensive floral and palynofloral records within the Cap
vegetative growth (the "turfing-in" model of aux-Os Member, and we di cuss its implication
Knoll et al. 1979; Niklas et al. 1980). Interspe for estimating actual plant diversity and ecosys
cific plant interactions consequently would have tem complexity.
been limited as well, and early land plants are
con idered to have exhibited relatively little
niche specialization (DiMichele et al. 1992). MATERIAL AND METHODS
They are further believed to have been re
su·icted by their free-sporing life cycle and rela More than 186 meters of stratigraphic section
tively inefficient water conducting systems to were logged in centimeter-scale detail along sea
moist, mesic sites, unable to occupy high-stress cliffs exposed near d'Aiguillon, Quebec (figure
(e.g., hydric or xeric) habitats (Scheckler l 986b; 11.1). The strike and dip of the strata are such
Bateman 1991; Algeo and Scheckler 1998). that lateral exposure of most stratigraphic units
ome authors, nevertheless, place them in salt is limited to a few hundred meters. However,
marshe , a physiologically stressful habitat erosional irregularities in the coastline provide
(Schweitzer 1983; Retallack 1990; Edwards and three adjacent exposures of one plant-rich mud
elden 1993). In our opinion, these proposi s tone interval near the ba e of the measured sec
tions are be t treated as null hypotheses about tion. Three sections of this interval were mea
early land plant ecology, to be tested by inde sured (our Seal Rock Landing, Seal Rock West,
pendent data. It should be kept in mind that this and First Cove West localities) to construct a
gencrnlized model is more applicable to early more accurate three-dimensional depiction of
stages of land plant evolution. By Emsian time, the strata (figure 11.1). Each rock unit was
land plants had diversified into at least three examined for grain si£e and color variation, pri
major tracheophyte clades (lycophytes, trimero mary sedimentary structures, pedogenic fea
phytes, and rhyniopsids sensu Kenrick and tures, paleocurrent indicator , vertebrate and
Crane l 997a) as well as, most likely, one or more invertebrate fauna! assemblages, and ichnofab
bryophyte clades, suggesting concurrent eco rics. Samples of critical rock types and plant
logical differentiation. bearing strata were prepared for petrographic
Our goal here is to test hypotheses about early examination and taphonomic and ichnologic
land plant ecology again t data derived from analysis at Binghamton University. Photomo
detailed biostratinomic analysis of the Cap-aux saics of key stratigraphic intervals were con
Os sediments. We first present data on floristics structed to facilitate description of lateral varia
and taphonomy, then a summary of edimento tion in the strata. Polaroid images annotated in
logical evidence for overall depositional envi the field were later combined with the photo
ronment, as well as more detailed description mosaics to generate lateral bedding diagrams
and interpretation of depositional environ and facies distribution information. Paleocur
ments of plant mega.fossil assemblages. We have rcnt orientations were corrected for tectonic tilt
chosen to address the following issues in this of strata using the stereonet method outlined in
Early Terrestrial Plant Environments 181
Lindholm ( 1987). The sedimentary facies rec spores with dispersed spores, sporangia were
ognized were compared with tho e in modern mounted with no additional treatment on stubs
edimentary environments and comparable for SEM. For light microscopy (LM), some spo
ancient deposits, to interpret depositional envi rangia were subjected to Schulze's solution fol
ronmental conditions directly as ociatcd with lowed by ammonium hydroxide, washed, and
plant habitats and burial settings. Only a brief mounted in glycerine jelly. Other sporangia
ynopsis of this work will be provided here. Full were mechanically crushed, bleached for about
description and interpretation of the Cap-aux 7 minutes, and mounted, to standardize com
Os Member facies will be published separately parison with dispersed spores.
(Griffing et al. 2000). Additional information on the sedimentol
Plant megafossils were collected from every ogy and paleontology is drawn from observation
productive horizon within the measured sec of laterally equivalent ediments exposed along
tion. A total of58 megafossil sites were collected. the beach west of Gros Cap-aux-Os in Forillon
Where possible, large blocks were quarried, but National Park and in correlative units exposed
in some ea es only relatively small hand samples on the south shore of Gaspe Bay (figure 11. 2).
could be collected. Identification of plant tax.a Of particular intere t is an extensive sandstone
from each site was made first on whole-rock unit on the south shore with abundant plant
specimens in the laboratory, after degaging if remains permineralized by CaC0 . This unit
3
necessary. A representative subsample from correlates with the Cap-aux-Os Member on the
each horizon was selected and bulk macerated north shore (McGregor 1973, 1977), and in this
in hydrofluoric acid for spore samples and to chapter it will be referred to as the permineral
identify additional small, fragmentary, and ized sandstone plant assemblage.
minor components of the horizon. Some iden
tifiable material from bulk macerations was sub
jected to additional oxidative treaunent, either REGIONAL GEOLOGICAL SETTING
with Schulze's solution (saturated potassium
chlorate in concentrated nitric acid) or warm The Battery Point Formation is part of the Gaspe
dilute potassium hydroxide. In many cases, nat Sandstone Group, a coar ening-up elastic wedge
urally retted material was ufficient for observ developed on the southern edge of Laurus ia,
ing cellular details. Selected specimens were 10° to 20° S latitude in Emsian times (Scotese
dried from 100 percent ethanol and mounted and McKerrow 1990). As a consequence, cli
on stubs for observation and photography on mate was presumably tropical (Witzke 1990). In
Hitachi S-570 and Leica Stereoscan 440 scan the absence of reliable biological climate mark
ning electron microscopes (SEM). er , sedimentology alone can contribute little to
The digested material from every bulk mac understanding of climatic conditions du1ing the
eration was prepared for dispersed spores deposition of the Battery Point Formation,
(heavy liquid separation, residues bleached for except that rainfall was moderate,judging from
6 to 8 minutes, and mounted in glycerine jelly). the absence of calcrete and edimentological
In addition, every unoxidized horizon within evidence for perennial rivers. The Battery Point
the measured section was collected and pre Formation is exposed on both north and south
pared for dispersed spores. Approximately 200 shores of Gaspe Bay (figure 11.2); however,
to 600 grains were counted and identified per detailed correlation between strata on the two
ample. Information on dispersed spore assem shores is currently not possible because of facies
blages in this paper is based on 60 samples from differences. On the north shore, three members
the lower half of the measured section. A pale have been recognized (Brisebois 1981). Our
oecological analysis of the dispersed spore measured section is from the middle, mudstone
assemblage of the Cap-aux-Os Member will be dominated Cap-aux-Os Member, as it is richest
pre ented elsewhere. For comparison of in situ in plant fossils (figure 11.2). The age of the
\
York River Fonnotion
- ------ -------------
Petlt:G~~ ;,~m~-;
--
------ \'
undifferentiated Cap du Petit-Gasp<!
upper Battery Point
Gros Cai>-awt·Os
0
CAP-AUWS e
km
0
Figure 11.1.
Map of study area, depicting distribution of members of Battery Point Formation, and measured sections on the north side of Gaspe Bay (after Lawrence 1986). Sec-
tions referred to in text: D'A, d'Aiguillon; MD5, Mudstone 5; SSE, Sandstone E; FCW, First Cove West; SRW, Seal Rock; SRL, Seal Rock Landing.
EarlyTerrestrial Plant Environments 183
Battery Point Formation
South Shore North Shore
Upper rt'\
PFreovret t ( uBnadtitfefc rryc nPuoaitncdt ) ·ac0:
Member -~
/ <
/
------1/
Cap-aux-Os Sandstone 3
Member
Sandstone 2
Interval
Petit-Gaspe
Lower /detailed in
Member
Battery Point this study
Mst. 5
(undifferentiated)
I c:
I ·a"0 ' ~MSss ts.34 t.D
I -~ Sandstone C Mst. 2 E:OO·
I
<
Sandstone 8
I
______ ,!
Mst. I
Figure 11.2.
Inferred correlalion between exposures of Battery Point Formalion on the north and south shore of Gaspe Bay, and
generalized measured section on north shore of Gaspe Bay, based on the internal straligraphy of Lawrence (1986) and
the study described in this chapter. Stippk, red mudstone intervals; black, gniy mudstone intervals.
Battery Point Formation is early Emsian to early caulis, Sawdonia, and Sciadophylon. Zosterophylls,
Eifelian, based on data from brachiopods even as fragments, can be identified to species
(Boucot et al. 1967) and spores (McGregor 1973, level on the basis of their distinctive cuticle.
1977; Richard on and McGregor 1986). The Trimerophytes are readily recognized as a group
measured section falls within the early to late from fragmentary vegetative material, although
Emsian annulalus-sexlanlii spore assemblage identification to species level requires relatively
zone of Richardson and McGregor ( 1986). complete specimens. Two other organisms, Pr<r
totaxites and Spongi.ophyton, are also important
constituents of the flora. We interpret both as
FLORISTICS AND TAPHONOMY eumycetous fungi. Protolaxites has recently been
reinterpreted as a gigantic perennial fungal body
Approximately 18 species of embryophyte analogous to a modem bracket fungus (Hueber
megafossils have been recognized within the 1996). Anatomically preserved specimens of
Cap-aux-Os Member, including two possible Spongi.ophyton from the south shore permineral
bryophytes, two probable gametophytes, six zos ized sandstone assemblage, consisting of septate
terophyll species, one lycopsid, two species of hyphal tubes surrounded by a thick resinous
rhyniopsid, and at least five species of trimero "cuticle" ofapparentlyagglutinated hyphae, have
phytes (table 11.1). Details of the taxa may be been interpreted as lichens (Stein et al. 1993).
found in the appendix to this chapter and the Consideration of taphonomy of fossil assem
associated figures. Common taxa include Psilo blages is essential to any reconstruction of their
phyton, Pertica/ Trimerophyton, Huvenia, Crenali- growth environment. As emblages reflecting
184 Early Terrestrial Plant Environments
Table 11.1. Megaflora of Cap-aux-Os Member
and associated dispersed spore taxon (where known)
Plant megafossil Dispersed spore taxon
Embryophyta incertae sedis
New genus A (?hepatic)
Sciadophyton spp. N/A
ewgenus B N/A
ew genus C
Rhyniopsida
1-/uvenia sp. nov. Retwotriletes or Calamospora
"Taeniocrada dubia" (new genus 0) ?
Lycophytina
Drepanophyws s/1in(u'f<mnis Goeppert R.etusotrill'tes or Cr1la11w~por«'
Crmaticaulis Vl'TT!tcttlosus Banks et Davis R.etusotriletes spp.
Rmalia hueberi Gensel R.etusotriletes spp.
Sawdonia omata (Dawson) Hueber R.etusotriletes spp.
'losterophyllum sp. R.etusotri!Ltes spp.
ew genus E ?& tu .sotri!Ltd'
"Bathurstia" sp. (new genus F)
Trimerophytes
Ps1loph)'ton dawsonii Banks, Leclercq et Huebcr A/J1culiretusispora amwrugosa morphon
I~ forbesii Andrews, Kasper et Mencher A. arenomgosa morphon
R pnnceps (Dawson) Hueber A. arenorugosa morphon
Pertica varia Granoff, Gensel et Andrews A. amwrugosa morphon
Pertica sp. A. arenorugosa morpho11
Trimeroph)'ton robu.stius (Dawson) I lopping A. arenorugosa morpho11
?, unknown, or questionable; I A, not applicable.
• pores have been extracted from fer·tile specimens of Drepanophycus spinaefonnit from ew Brun~wick (Li et
al. 2000).
h Ba.o.ed on spore masses from a monodonrina111 a~st'mblage of new genus E.
' RC'ported by other authors but not found in this study.
minimal transport [autochthonous or parauto free to become aligned to water currents. Regu
chthonous-i.e., transported but still within life lar spacing of aerial axes, especially characteris
environment (Bateman 1991)] are essential to tic of large trimerophytes, again suggests attach
interpret growth habitat, habit, and associa ment to a horizontal stem or rhizome that is no
tions. We judge that roughly 35 of the 58 longer preserved. That axes branch through the
megafossil assemblages in the Cap-aux-Os Mem bedding plane is another strong indicator of in
ber arc either autochthonous or parautochtho situ burial, since this position is not energetically
nous. Evidence of minimal transport includes stable under most conditions of transport.
evidence of anchorage, axes oblique or perpen Examples of this include aerial axes of Sawdonia
dicular to bedding plane, and preservation of ornala at the type locality, and stalked game
complete axes with fine detail. Other authors tophores of SciadofJhyton and new genus B that
have noted that early land plant assemblages are extend through the bedding plane into superja
often preserved in parallel alignment along the cent sediments. The preservation of delicate
bedding plane, as though plants were rooted at structure such as distal branch tip , buds, and
one end in the process of burial by the enclo - attached sporangia suggests limited transport,
ing sediment (Andrews et al. 1977; Edwards although the degree of resistance to fragmenta
l 979a). Ob ervation of axes undulating in par tion and degradation that these plants may have
allel suggests that they were anchored (presum displayed is unknown because of the lack of
ably rooted) at one end, leaving the other ends taphonomic analyses of appropriate analogue .
Early Terrestrial Plant Environments 185
Most important, among the major classes of tra lated lingulid brachiopods (figure ll.40); and
cheophytes identified from t.he measured sec (5) dark gray shales and siltstones bearing
tion (lycophytes, trimerophytes, rhyniopsids), acritarchs, small bivalves, or small, thin-shelled
none appear to have exhibited significant dif lingulid, ort.hid, and rhynchonellid brachiopods.
ferences in preservation potential.
Overall Environmental Interpretation
SEDIMENTOLOGY
The Cap-aux-Os Member is interpreted as flu
vial and delta-plain deposits (Lawrence 1986;
Fades Associations
Lawrence and WilJiams 1987; Bridge et al. 1998;
Lawrence (1986) recognized three facics associ Griffing et al. 2000). Paleocurrents suggest that
ations within the Cap-aux-Os Member (figure the rivers transported sediment seaward to the
11.2). The lowermo t of these as ociations northwe t. Whereas Lawrence (1986) inter
(Association 1) contains 4- to 60-m-thick, multi preted most of the multistory sandstone bodies
story sandstone bodies separated by thinner, in Association I as deposits of high-sinuosity
red- and gray-mudstone-dominant intervals. river channels, marine or marginal-marine trace
Association 2 is dominated by gray mudstones at fossils (i.e., Diplocraterion) in the upper portions
d'AiguilJon but al o contains decimeter- to of Lawrence's Sandstones B, C, and D (As ocia
meter-thick andstone sheets, and lenses and tion 1, figure 11.2) suggest close proximity to
meters-thick, single-story sandstone bodie . Rel the sea. Furthermore, the single-story sandstone
atively coarser-grained, multisto1-y sandstone bodies in Association 2 strongly suggest deposi
bodies with uncommon, thinner red mudstone tion in the form of channel bars and fills very
intervals characterize the uppermost association close to the tidal limit of tidalJy influenced chan
(Association 3) of the Cap-aux-Os Member. nels (Bridge et al. 1998; Griffing et al. 2000).
Paleocurrent indicators within the sandstones Features of the mudstone-dominant intervals of
in the measured ection al d'Aiguillon suggest Associations 1 and 2 just listed also suggest
unidirectional flow to the north and west, but a a coastal plain setting, with levees and creva se
few strata indicate opposing paleoflow direc splays, freshwater marshes and lake , lacustrine
tions (Lawrence 1986; see following paragraph deltas, brackish marshes and interdistribu
and figure 11.3). Most of the in situ and in loro tary bays, and sandy and muddy tidal flats (GrifT
plant megafossil horizons lie within the mud- ing et al. 2000). The ea terly directed pale
tone-rich portions of upper Association 1 and ocurrents are interpreted as flood tidal currents.
throughout Association 2 (figure 11.3), whereas These types of marginal marine deposits have
highly fragmented assemblages occur in the also been described from the Catskill elastic
multi tory sand tone bodies. wedge in New York and Pennsylvania (Bridge
M udstone-domi nan tin ten-<tls contain a variety and Droser 1985; Halperin and Bridge 1988;
of distinctive features, including (1) meters-thick Bridge and Willi 1994).
sandstone sheets and lenses that fine or coar en
upward, and that indicate paleoflow directions
Plant-Bearing Fades
to the northeast to outheast, as well as to the
north and west; (2) centimeter-scale, lenticular, Most in situ and in loco plants in the Cap-aux-Os
wavy, and flaser bedding with wave-ripple and Member lie within two types of facies sequence:
current-ripple marks (figure l l.4A); (3) cen (1) at or near t.he base of 1- to 2-meter-thick,
timeter- to decimeter-thick sandstone sheets coarsening-upward sandstone sheets or lenses
containing wave-ripple marks, Dipwcraterion-, (Facie A), and (2) within the uppermost por
Skolithos-, and Phycodes-1ike traces, and disarticu tion of 4- to 5-meter-thick, fining-upward
lated cephalaspid fish skeletons (figure l l .4B,C); single-story channel sandstone bodies (Facies
( 4) desiccation-cracked mudstone with articu- B). Examples of both types of sequence occur
186 EarlyTerrestrial Plant Environments
I 16
-T flood tidal delta
(single event)
I
tidal channel
15 ::::: ebb tide deposit
I
- -& ::::-
flood tide
deposits
/
--
14
lnterdistributary lake
(little or no communication deep lntenllstributary
w/ HA) bay or prodelta
- brackish to marine bay
(washover sands ?)
13 21
s~~~ ---
- - --:::. tidal channel or flood basin or
- tide-Influenced wet floodplain
""-.; - dlstributary channel with paleosols levee with paleosols
(proximal to mouth)
4 -
__..,, cross-bar
channel
___...- ./
1' "" 19~~~
11 :;::"" --
_....... IN srru \"-.. distal to medial bar top tevee
........,.....,..-.., PUNTS " crnasse splay / levee
Sa"'®niD ~ shallow standing
water (swamp)
cross·b ar
fluvlal channel
2 (distrlbutary?) 10 18 / channel
- flood basin or
wet floodplain
with paleosols
9 17
Cl> • UMUUDS flood tide
deposits
tidal sand flats
8
vrrmc vc v((mcvc vffmcvc
clay slit sand clay slit sand clay silt sand
Figure 11.3.
Detailed log of base of Association 2 (top of Association l is at 3 m mark). See text for further details. See p. l 87 for
key to symbols used.
within the lowermost 19 meters of Lawre Description of Fades A
nce's Association 2 section at d'Aiguillon: the
Sawdonia omata type (Hueber 1971) and The type Sawdonia omata horizon is situated at
the Pertica varia type localities (Granoff et the base of a coarsening-upward sequence of
al. 1976), respectively (figure 11.5). In ad dark green-gray, silty claystones, to light green
dition, a distinctive transported assemblage of gray, fine-grained sandstones (figure l l.3A,
Spongi.ophyton and Prototaxites occurs within meters 10 to 12), like several other in situ zos
the fine-grained portions of the multistory terophyll occurrences in Association 1 and 2.
channel sandstone bodies in Associations 1 This sequence is 1. 7 m thick at the base of the
and 3 (Facies C). exposure and tapers laterally upslope to about 0.5
Early Terrestrial Plant Environments 187
l(EY
Tcctonizcd suata
__... Unidirectional palcocurrenl orientation
Fine blocky weathering (thin-lines) with
-.._ Bidirectional paleocurrent orientation
pscu<loanticlinc stmcturcs (thick-lines)
Rc<lox color mottling
~ Surfnce trails and tracks
Polygonal mu<lracks
Lenticular bcd<ling wilh unidirectional ripples U~ 11[ Skolitlros -like vertical burrows ("piperock")
Wavy bcd<ling wilh unidireclional ripples
Haser bcd<ling with unidirectional ripples
U-shapc<l burrows
Indistinct bedding
Wave (oscillation) ripple cross-slratilied
Phycodes-like stellate burrows
Lenticular bedding wilh oscillation ripples
Wavy bedding wit..h oscillation ripples
Burrow homogenized beds
Haser bedding wilh oscillation ripples
Fish skelelal fragmenlS
Herringbone cross-slratification
Climbing ripple cross-sLratific.1tion witJ1 Pterygotus eurypterid fragments
concretionary cementation
Planar sLratificatiou Small bivalve -c;icardella
Small-scale liough cross-slratific.1tio11
Medium-scale planar cross-strntilicatio11 Small bivalve -Modiolopsis
Medium-scale trough cross-stralilicatio11
Irregular horizonlal to sub-vertical organic
Scour overlain by intrafomrntional clasts
bands (fossilized plant rhi1.0mcs?)
Large plant fragmenlS
Large-scale cross-stratification /11 silu plant megafossils
(lateral accretion) surface
"Gamctophytc establishment surfaces"
m thick (figure 11.5). Dusky red-gray siltstones occurs at the base of the plant accumulation; it
and dark green-gray mudstones underlie the aw resembles the underclays of Carboniferous
donia horizon. The dusky red-gray siltstones have cyclothems in North America and Europe
blocky fabric with slickensides and weakly devel (Dulong and Cecil 1989). Waxy, dark green-gray
oped pseudoanticline structures (i.e., vertic pale mudstones above the underclay contain dense
osol features). These siltstones contain car accumulations of toppled Sawdonia axes. These
bonized, boudin-like structures that lie parallel to mat-like accumulations of horizontal axes extend
one another on the bedding plane but produce along the entire outcrop and create localized
short lateral branches that plunge several mil paper coals (figure l l.6B).
limeters into the bedding plane (figure l l.6A). A sheet-like to lenticular, coarsening-upward
We tentatively interpret these as plant rhizomes. sandstone body directly overlies the Sawdonia mat.
Overlying the rhizome-bearing beds are dark The lowermost 30 cm of this body grade upward
green-gray mudstones that contain weakly devel from muds tone to small-scale cross-stratified, fine
oped blocky fabric (also interpreted as paleosol grained sandstone. Numerous anchored plant
features; ee also Elick et al. 1998b). A discontin axes that protrude upward through the siltstones
uous, extensively weathered claystone interval and sandstones indicate unidirectional flow to
188 Early Terrestrial Plant Environments
before curving upward again. In silu Sawdonia
axes terminate approximately 35 cm above the
clay seam at the base. This coincides with the
coarsest sandstone ·within the unit. Axes of a
large trimerophyte with laterals stripped, and
fragments of Sawdonia, Crenalicaulis, liJsterophyl
lum p., Drepanophycus, and other taxa are com
mon within the upper portion of the coarsen
ing-upward sequence. These very fine to
fine-grained sandstones are dominated by
climbing-ripple cross strata (angle of climb up
tol3°).
The coarsening-upward sandstone body is
directly overlain by (l) fining-up, dusky red-gray
siltstones with light green redox mottling and
blocky fabric; (2) dark green-gray mudstoncs witl1
weakly developed blocky fabric; (3) decimeter
thick beds of dusky red and grecn-gray siltstone
featuring Slwlithos and U-shapcd traces, and disar
ticulated fish bone; and ( 4) a 32-<:m-thick bed of
dark gray shale with wave-rippled lenses of silt
stone, containing numerous acritarchs at the base
and millimeter- to centimeter-sized bivalves
(?Cyprirardella) throughout (figure 11.3).
We have identified at least 10 additional in silu
plant horizons associated with tl1is type of coars
ening-upward equence. Like the type Sawdonia
sequences, mosl of t11ese coarsening-upward
Figure 11.4. sheet5 and lenses vary from decimeters to meters
A: Wave-rippled naser-bedded andstones, showing nel in thickness. One such sequence in Mudstone 4,
migration direction (leftward) opposile 10 thal of Association 1 ( cc figure 11.2) has a triangular
underlying medium-scale tabular cross-s1rata, Associa
cross ection. The upper portions of some of these
tion 2 al d'Aiguillon. ( 1 cm scale, lower left.) B: Bedding
sequences also include features not displayed in
plane view of a muddy siltstone containing Skolithos-like
vertical tube Lraces (white spots) and desicca1ion-crack t11e Sawdonia example, such as meter-scale chan
polygons, Lawrence's Mudstone 4, Associa1ion 1 at expo nel forms, wave ripple cross-stratification, and
sures on Lhe west side of Seal Rock Poim (SRW) (I dm
extensive disruption by burrows.
scale). C: Upper surface ofa while lO buff medium sand
stone bed displaying protrusive U-shaped burrow iden
tified as Diplocraterion, Lawrence's Mudstone 4, Associa
Interpretation of Fades A
tion I al First Cove West (FCW) west of Seal Rock Point
(I dm scale, lowerkft). D: Articula1ed but transported lin Environmental interpretations of the lower
gulid brachiopods forming one of several thin concen
d 'Aiguillon section are provided with the strati
trations within gray mudstones with large polygonal
graphic log (figure 11.3). Basal mudstones dis
mudcracks, lower portion of Lawrence's Mudstone 5
inte1-val, Association 1 east of d 'Aiguillon. ( 1 cm scale-, play features typical of floodbasin muds with
lower kft.) weakly developed paleosol horizon (e.g., verti
sols, inceptisols, and entisols: see also Elick et al.
the east and southeast (figure l l.6C). Some of l 998b). The underclay and waxy clays tone
the longer axes display several abrupt bends lo underneath the Sawdonia mat and other plant
a shallower angle at the base of sandstone beds, assemblages most likely represent swampy gley
