TRACE FOS SILS FROM THE EOCENE LILLEBLT CLAY
FOR MA TION, RSNS PEN IN SULA, DEN MARK
Jan Kresten NIEL SEN
1, Jesper MILN
2, 3& Dan iel MESFUN
41
VNG Norge, Filipstad Brygge 1, NO-0252 Oslo, Nor way; e-mail taphofacies@hotmail.com
2
Geomuseum Faxe / stsj³llands Mu seum, stervej 2, DK-4640 Faxe, Den mark
3
Nat u ral His tory Mu seum of Den mark, ster Voldgade 5-7, DK-1350 Co pen ha gen K, Den mark
4
GXU - Gladsaxe 10. Klasse og Ungdomsskole, Gladsaxevej 198, DK-2860 SÝborg, Den mark
Niel sen, J. K., Mil´n, J. & Mesfun, D., 2015. Trace fos sils from the Eocene Lilleb³lt Clay For ma tion, RÝsn³s pen in sula, Den mark. Annales Societatis Geologorum Poloniae, 85: 493–505.Ab stract: A cliff ex po sure of the Eocene Lilleb³lt Clay For ma tion, on the RÝsn³s pen in sula of Zea land, Den -mark, has yielded a di verse trace-fos sil as sem blage. The trace fos sils are de scribed for mally for the first time and as signed to Phymatoderma melvillensis, un named clus ters of small bur rows, Ophiomorpha nodosa, Spongelio-morpha isp., Dreginozoum beckumensis, Bichordites isp., Chondrites isp., Atollites zitteli? and ?Rhizocorallium isp. The pres er va tion of the trace fos sils is strongly re lated to early diagenetic en hance ment. The trace-fos sil as sem blage is dom i nated by a com bi na tion of dwell ing and feed ing bur rows. The bioturbation took place in very clean clay of a shelf set ting far off shore. The trace-fos sil as sem blage is in dic a tive of the dis tal Cruziana ichnofacies.
Key words: Ichnotaxonomy, bioturbation, ethol ogy, ichnofacies. Manu script re ceived 9 Jan u ary 2015 ac cepted 29 May 2015
IN TRO DUC TION
The RÝsn³s pen in sula in west ern Zea land, Den mark, is
geo log i cally a clas si cal area, which has been the sub ject of
sev eral in ves ti ga tions dur ing the last cen tury (Fig. 1). The
glacio-tectonical com plex ity and ice-move ment di rec tions
ev i dent in north west ern Zea land have been thor oughly dis
-cussed and re
viewed by Andersen (1964), Berthelsen
(1971, 1975), Petersen (1973b), Houmark-Niel sen (1981,
1987) and oth ers. Also, field ex cur sions have taken place
frequently in this area (e.g., Petersen, 1970). The pen in sula
com prises strata from the lst For ma tion, RÝsn³s Clay For
-ma tion, Lilleb³lt Clay For -ma tion and Qua ter nary de pos its.
It is the type area for the RÝsn³s Clay For ma tion. Petersen
(1969) found trace fos sils at the +103 ash layer of the Paleo-
cene lst For ma tion, close to the over ly ing Eocene RÝsn³s
Clay For ma tion. He briefly sub di vided the trace fos sils into
mor pho log i cal groups. No other ichnological stud ies have
been car ried out on the RÝsn³s pen in sula. The lo cal i ties in
in land quar ries ex am ined by Petersen (1969) are to day to
-tally over grown and in ac ces si ble. A few ac ces si ble out crops
are found in the coastal cliffs. A pre lim i nary sub di vi sion of
the trace fos sils into mor pho log i cal groups was done by
Niel sen and Mil´n (2014). The aim of this study is to for
-mally de scribe the trace-fos sil as sem blage of the Lilleb³lt
Clay For ma tion on the RÝsn³s pen in sula (Figs 1–3), and to
pres ent the view that the an i mal be hav iour re flected in the
trace fos sils sheds light on the depositional con di tions.
GEO LOG I CAL SET TING
A ma jor trans gres sion, the Ypresian trans gres sion, took
place in north west ern Eu rope af ter the ear li est Eocene (e.g.,
Ras mus sen et al., 2008). The Eocene strata of Den mark
comprise mainly very fine-grained de pos its of the RÝsn³s
Clay, Lilleb³lt Clay and SÝvind Marl for ma tions. These ac
-cu mu lated in a deeper ma rine en vi ron ment, far from the
pa-laeocoastline in North Ger many, South Swe den and South
Nor way (Gravesen, 1998). The Eocene ac cu mu la tions are
up to 200 m thick in west ern Den mark. Ow ing to Neo gene
up lift and Qua ter nary glaciations, the ac cu mu la tions were
eroded near the Fennoscandian Shield (Ras mus sen et al.,
2008).
The Lilleb³lt Clay For ma tion was formed dur ing late
Ypresian to early Lutetian (Schnetler and
Heilmann-Clau-sen, 2011). It con sists mostly of grey ish to green ish,
non-cal car e ous clay (Heilmann-Clausen et al., 1985). The clay is
of hemipelagic or i gin. Molluscs from the north ern Lilleb³lt
area be tween Funen and Jutland in di cate wa ter depths from
lower sublittoral to up per bathyal, equal to about 100–300 m
(Schnetler and Heilmann-Clausen, 2011). Fos sils of ma rine
crabs, squillae, pteropods and fishes have been found at var
-i ous lo cal -i t-ies (Col l-ins and Jakobsen, 2003; Janssen et al.,
2007; Schwarzhans, 2007; Bonde et al., 2008). The
Lille-b³lt Clay For ma tion is widepread in the Dan ish area of
Eo-cene de pos its, ex cept in north ern Jutland. The for ma tion is
lat er ally equiv a lent to a part of the Horda For ma tion in the
North Sea Cen tral Trough (Michelsen, 1994; SchiÝler et al.,
2007; Ras mus sen et al., 2008). The RÝsn³s pen in sula in the
west ern part of Zea land con tains glacio-tec toni cally
distur-bed in ter vals of the Paleocene lst For ma tion as well as the
Eocene RÝsn³s Clay and Lilleb³lt Clay for ma tions (see Pe-
tersen, 1973a, b, 1978; Berthelsen, 1975;
Heilmann-Clau-sen et al., 1985). The lithological units R5 and R6 of the
RÝsn³s Clay For ma tion are over lain by the Lilleb³lt Clay
For ma tion. The lat ter is rep re sented by the lithological units
L1, L2, L3 and L4 in coastal sea cliffs and clay pits pres ent
on the RÝsn³s pen in sula (Heilmann-Clausen et al., 1985).
The pres ent au thors in ves ti gated a sea cliff lo cated on the
south west ern coast of the pen in sula (Figs 1–3). The sea cliff
is about 520–575 m north-north west of the road Charlesvej.
It is about 15 m high and 55 m in to tal length. Unit L4 is
pres
ent in the north-north
west
ern 25 m part of the cliff
(WGS84: 55°43¢18²N, 10°59¢50²E). The lower part of unit
L4 com prises green ish to blu ish clay. The up per part con
-sists of green ish clay with brown ish clay beds and car bon ate
con cre tions. The mid dle part of the cliff is cov ered by 25 m
of scree de pos its. Unit L3 oc curs in the south-south east ern
5 m of the cliff (55°43¢17²N, 10°59¢51²E). This unit com
-prises red-brown clay, where car bon ate con cre tions are rare
near its mid dle. At this lo ca tion, unit L2 is partly pres ent
and is char ac ter ised by grey-green clay with nu mer ous thin
lenses of black clay.
Ac cord ing to Schnetler and Heilmann-Clausen (2011),
a dis tinct ho ri zon is pres ent within or at the base of unit L3.
The ho ri zon, which is bioturbated and ce mented, ex ists in
all Dan ish on shore oc cur rences of this unit. The ho ri zon is
most likely a con densed sec tion, the age equiv a lent of the
max i mum flood ing sur face at the Ypresian-Lutetian Global
Stratotype Sec
tion and Point (Schnetler and
HeilmannClausen, 2011). Con cre tions oc cur dis con tin u ously ap prox
-i mately -in the m-id dle of un-it L3 -in the sea cl-iff. These m-ight
in di cate the Ypresian-Lutetian bound ary.
The units of the Lilleb³lt Clay For ma tion are folded in
a com plex way and are char ac ter ised by densely oc cur ring
frac tures that lower the pres er va tion po ten tial and vis i bil ity
of trace fos sils within the sea cliff. This com plex ity is re
-lated to glacio-tec tonic com pres sion as well as late gla cial to
postglacial land slide events (Berthelsen, 1975, p. 13–14).
MA TE RIAL AND METH ODS
The Lilleb³lt Clay For ma tion is poorly ex posed in the
sea cliff in ves ti gated. The cliff is partly cov ered by veg e ta
-tion and dif fi cult to ac cess dur ing pe ri ods of rainy weather.
Ob ser va tion of trace fos sils in situ is there fore chal leng ing
in the field. Spec i mens, par tially or en tirely pre served as iron-
stone con cre tions (see Huggett, 1993; Gravesen, 1995), were
hand-picked from the out crop it self and the scree de pos its.
Some spec i mens were cross-sec tioned and pol ished by us ing
an or di nary drill ing ma chine equipped with di a mond cut ting
and pol ish ing discs. The spec i mens were clas si fied sys tem at
-i cally, fol low -ing the -ichnotaxobase gu-ide l-ines by Bertl-ing et
al. (2006). All the spec i mens are lodged in the col lec tions of
stsj³llands Mu seum as un num bered items.
RE SULTS
Sys tem at i cal ichnology
Ichnogenus Phymatoderma Brongniart, 1849
Phymatoderma melvillensis Uchman and GaŸdzicki, 2010
Figs 4, 5
Ma te rial: 45 spec i mens (unit L4, out crop scree).
De scrip tion: All spec i mens are pre served in full re lief, de rived Fig. 1. Lo ca tion of in ves ti gated sea cliff with the Lilleb³lt Clay For ma tion in the RÝsn³s pen in sula, Den mark. The Eocene at the pre-Qua ter nary sur face is in di cated by dark grey shad ing (af ter Sorgenfrei, 1939; Gravesen, 1998).
from bur row sys tems with ir reg u larly bi fur ca tion. The an gle of bi -fur ca tion is acute, typ i cally less than 45°. The bur row di am e ter is 13.4–18.6 mm. In di vid ual spec i mens are fairly con stant in di am e -ter. Branch ing is ir reg u lar, typ i cally as first-or der side branches from the main bur row. Branches have round or ta pered ends. Spec -i mens pre served -in s-itu are pref er en t-ially or-i ented hor -i zon tally to slightly obliquely. The bur row fill, which is dark grey to black, con tains pel lets that are oval to cir cu lar in cross-sec tion. The length and width of the pel lets are 2.0–3.1 and 1.0–1.8 mm, re -spec tively. The pel lets are vi su ally most dis tinct on the bur row mar gin, which they tend to par al lel. In ter nally the pel lets ap pear to be ran domly dis trib uted.
Re marks: These fos sil ised bur rows are most com mon in the out -crop of the Lilleb³lt Clay For ma tion in ves ti gated. Be cause of
com pac tion, their orig i nal ori en ta tion may have been more ver ti cal. The bur rows are pre served as iron stone con cre tions. As in di -cated by a bright yel low ish col our, the in fill ing of the bur rows can lo cally con tain weath ered py rite. The oc cur rence of py rite in the Lilleb³lt Clay For ma tion was also re cog nised by Huggett (1993) and Gravesen (1995). Pyritisation pos si bly oc curred in the mu cus lin ing, se creted by the tracemaker; Simpson (1957) ten ta tively sug gested this for Chondrites.
Sim i lar trace fos sils are il lus trated un named by Gravesen (1995, p. 18), pre served as clay-iron stone con cre tions from Trelde N³s in east ern Jutland. His il lus tra tion con firms the ir reg u lar branch ing pat tern and pel let dis tri bu tion. Fu (1991) pro vided a de scrip tion and com pre hen sive syn onym list for Phymatoderma granulata (von Schlotheim, 1822), which is branch ing in a Chondrites-like man ner, with feath er ing ap pear ance of probes (Fu 1991; Seilacher 2007; see von Schlotheim, 1822, pl. 5, fig. 1). Phymatoderma melvillensis Uchman and GaŸdzicki, 2010 is dis tin guished by a lo -cal meniscate struc ture with pel lets and its branch ing pat tern. The menisci are weak in ap pear ance and can be ab sent in some spec i -mens (Uchman and GaŸdzicki, 2010). A sim i lar branch ing pat tern oc curs in the spec i mens from the RÝsn³s pen in sula. Uchman and GaŸdzicki (2010) also dis cussed the dif fer en ti a tion from other ichnotaxa. More re cently, García-Ramos et al. (2014) de fined the ichnogenus Tubotomaculum García-Ramos et al. for hor i zon tal, spin dleshaped bur rows, dis play ing teichinoid spreite and el lip soi -dal pel lets. Even though the pel lets are sim i lar in shape, Tuboto-maculum dif fers from P. granulata and P. melvillensis in bur row out line and the lack of bi fur ca tion.
Clus ter of small bur rows
Fig. 5
Ma te rial: 6 spec i mens (unit L4, out crop scree).
De scrip tion: This struc ture is a bul bous to elon gate lump, char ac -Fig. 2. Strati graphi cal over view and li thol ogy of units in the
Lilleb³lt Clay For ma tion, RÝsn³s pen in sula. Mod i fied from Heilmann-Clausen et al. (1985, fig. 11).
Fig. 3. Cliff ex po sures of the Lilleb³lt Clay For ma tion. A. Lithological unit L4. B. Units L3 and L2.
Fig. 4. Phymatoderma melvillensis from the Lilleb³lt Clay For ma tion. A, B. Side views. C. Cross-sec tion with diagenetic zonation. Thin yel low ish zone is prob a bly weath ered py rite. D. Diagenetic en hance ment of lower part of pelletal bur row fill, the up per part is ab -sent. Up per view and cross-sec tion.
ter ised by an ir reg u lar out line. The lump is an elon gate fea ture around a branch of Phymatoderma melvillensis, or it is pres ent as a sub-spher i cal fea ture at the mar gin of such a branch. The lump com prises a com plex ity of small bur rows. These bur rows are cy lin dri cal in out line and they are ir reg u lar in cur va ture and ori en ta -tion and bi fur cate at acute an gles. The bur row di am e ter is about 3.0–4.5 mm. The bur row fill is structureless and dark grey to black, while the ma trix be tween the small bur rows is light grey. Re marks: Mod ern, anal o gous struc tures were de scribed by Brom- ley and Frey (1974) and Frey and Howard (1975) as ter mi nal cham bers, from which min ute bur rows ex tend. Frey and Howard (1975) in ter preted the bur rows as pos si bly ex ca vated by young post-lar vae of the deca pod crus ta cean Upogebia affinis (Say). The cham bers may have con tained or ganic mat ter uti lised by the ju ve niles. Also, Forbes (1973) de scribed how ju ve niles of an other bur -row ing crus ta cean Callichirus kraussi (Stebbing) meta mor phose to the post-lar val stage in side the par ent bur row. Af ter wards, they dig into the bur row wall (Forbes, 1973). This is a prob a ble mode of or i gin for the clus ters from the Lilleb³lt Clay For ma tion. Also, they could have emerged as ac tual hatch ing struc tures made by the off spring. For in stance, D’Alessandro and Bromley (1995) char ac -ter ised Pleis to cene Spongeliomorpha sicula D’Alessandro and Bromley as hav ing two types of ovoid cham bers, (A) subspherical struc tures at and be low hor i zon tal maze bur row sys tem and (B) ver ti cally elon gated struc tures ris ing above the maze. The cham -bers were prob a bly made by crus ta ceans for mi cro bial gar den ing (agrichnion), or breed ing (D’Alessandro and Bromley, 1995). The clus ters could oth er wise be a re sult of the ac tiv i ties of sec ond ary tracemakers that ex plored the bur row fill for or ganic ma te -rial, as a commensal in ter re la tion ship. How ever, the clus ters are not as so ci ated with any kinds of trace fos sils other than P. melvillensis. Also, the clus ters may oc cur rather mar gin ally to the bur -row fill of P. melvillensis.
Crowded tan gles of tor tu ously branched tubes oc cur in the Ju ras sic Sorthat For ma tion (Bromley and Uchman, 2003). The tan -gles, Bornichnus tortuosus Bromley and Uchman, are ovoid and a few centi metres in size. A dis tinct wall lin ing dis tin guishes them from the clus ters in the Lilleb³lt Clay For ma tion.
Ichnogenus Ophiomorpha Lundgren, 1891
Ophiomorpha nodosa Lundgren, 1891
Fig. 6A, B
Ma te rial: 1 spec i men (out crop scree).
De scrip tion: The ichnotaxon is rep re sented by a sin gle bur row seg ment that is dis tinctly lined with pel lets of ag glu ti nated muddy sed i ment. The pel lets are glob u lar in out line and about 4 mm across. The pel lets ap pear as nu mer ous pro tu ber ances (knobby) in the outer sur face of the wall lin ing. The wall lin ing is smooth on the in side. There is no bur row fill in side. Bi fur ca tions have not been re cog nised in the pres ent spec i mens.
Re marks: The rec og ni tion of O. nodosa in the pres ent ma te rial is based on the prom i nent wall lin ing and the cy lin dri cal cir cum fer -ence of the bur row, which are di ag nos tic char ac ters for the ichno-taxon. Branch ing of the bur rows is also in cluded in the di ag no sis of Ophiomorpha (e.g., Fürsich, 1973, 1974; Bromley and Frey, 1974; Niel sen et al., 1996 and ref er ences therein); how ever, it is not pres ent in the spec i men stud ied. The ma te rial by Lundgren (1891; see Andersson, 1981) is un branched, ex cept for one spec i -men. The au thors there fore as sign the spec i men with con fi dence to O. nodosa.
The scar city of spec i mens is prob a bly re lated to the gen eral lack of a need for bur row re in force ment in the clay-rich suc ces sion. The pel lets were formed dur ing sed i ment sort ing by the trace-maker, us ing its mouth parts. Gen er ally, the wall lin ing of O.
nodosa tends to be pre served as a concretionary halo, where the in te -rior and sur round ing sed i ment is less con sol i dated (Seilacher, 2007, p. 54). The pres ent spec i men shows a sim i lar way of pres er -va tion.
Fig. 5. Trace fos sils from the Lilleb³lt Clay For ma tion. A. Spher i cal clus ter of ir reg u lar smaller bur rows as so ci ated with Phy- matoderma melvillensis, side view. B. Up per view. C. Lower view.
Ichnogenus Spongeliomorpha Saporta, 1887
Spongeliomorpha isp.
Fig. 6C–E
Ma te rial: 5 spec i mens (units L4 and L3, out crop scree).
De scrip tion: A cy lin dri cal bur row sys tem with a smooth to stri ated, un lined bur row mar gin. The striation is pres ent as ridges ori -ented lon gi tu di nal to oblique with re spect to the main axis of the bur row. The bur row fill is structureless. The bur row is oval in cross-sec tion, about 25–35 mm wide. The di men sions are fairly uni form in in di vid ual bur rows. The bur rows are lin ear to curved in out line. Bur rows found in situ are all hor i zon tally ori ented. Re marks: The spec i mens are oval in ver ti cal crosssec tion, be -cause of the orig i nal bur row ing di men sions or par tial com pac tion. The lat ter would be in con trast to the in di ca tion of stiff mud by the striations, which are in ter preted as scratches made by the tracemaker (bioglyphs). The sub strate was suf fi ciently firm for wall re in force ment not to be re quired. The bioglyphs are com monly in -dic a tive of a muddy firmground (e.g., Seilacher, 2007; Gibert and Ekdale, 2010). Spongeliomorpha can clearly be dif fer en ti ated from Thalassinoides by its bioglyphs (Gibert and Ekdale, 2010).
The sys tem atic val i da tion of Spongeliomorpha, Ophiomorpha and Thalassinoides at ichnogeneric level has been ex ten sively dis
-cussed by Fürsich (1973, 1974), Bromley and Frey (1974), Frey et al. (1978), Schlirf (2000), Melchor et al. (2010) and oth ers. Wall struc tures, such as lin ing and striation, are con sid ered as valid taxobases, among oth ers, for ichnogeneric dis tinc tion (e.g., Mel-chor et al., 2010). The spec i mens de scribed herein are as signed to an un spec i fied ichnotaxon of Spongeliomorpha isp. and do not show a re pet i tive pat tern of bioglyphs. Its ir reg u lar ity in di cates that the pur pose of the bur rows was dwell ing; de posit feed ing is not ev i dent. A sim i lar in ter pre ta tion was reached by Gibert and Robles (2005) for Mio cene S. sudolica (Zarêczny) in the VallÀs-PenedÀs Ba sin, Spain.
Ichnogenus Dreginozoum Marck, 1894
Dreginozoum beckumensis (Marck, 1858)
Fig. 7A–C
Ma te rial: 4 spec i mens (unit L4, out crop scree).
De scrip tion: Subhorizontal, un branched bur rows, char ac ter ised by a me dian string about 1–2 mm wide. Biserially ar ranged lobes are placed lat er ally along the string. The lobes are like cof fee beans in out line, up to about 16 mm lat er ally out from the string. They are up to about 11 mm, mea sured par al lel to the string. Over -all bur row width is up to 33 mm.
Re marks: Häntzschel (1964) ten ta tively com pared Dreginozoum with egg cap sules of ma rine prosobranch gas tro pods; Dreginozoum was con sid ered as un re cog nis able (Häntzschel, 1975). Spec -i mens from the Lower Eocene of Ger many were stated by Re-ich and Klafack (2002) to be fos sil ised egg cap sules, but the tapho-nomical pro cess was not clar i fied to jus tify the three-di men sional pres er va tion of soft tis sue dur ing de cay and com pac tion. They re -jected an in ter pre ta tion as trace fos sils. In con trast, Kappel (2002) com pre hen sively ar gued Dreginozoum to be a monospecific ich-nogenus on the ba sis of spec i mens found at bed ding in ter faces. The lobes, which re sem bled cof fee beans in out line, con tained back fill struc tures re lated to ac tive grain sort ing (Kappel, 2002). The pro cess of back fill ing was sup ported by Seilacher (2007), who con sid ered Dreginozoum as the hypichnial pres er va tion of nereitid traces on sole sur faces.
Ichnogenus Bichordites Plaziat and Mahmoudi, 1988
Bichordites isp.
Fig. 7D
Ma te rial: 3 spec i mens (unit L4, out crop scree).
De scrip tion: Hor i zon tal, un branched bur rows, which are tu bu lar in out line and slightly oval in cross-sec tion. The width is up to 27 mm. The bur row shows a meniscate back fill that is par tially ce -mented. In di vid ual menisci are slightly cres cent and about 6 mm thick. The menisci are pen e trated by a weakly de vel oped string. The string is lo cated a lit tle be low the cen tre of the bur row. Pre -served in full re lief. The sub strate around the string is com monly more heavily min er al ised, form ing a preservational core. Re marks: The par tial ce men ta tion of the back fill menisci may be due to biogenic grain sort ing; how ever, the sed i ment con sists of very clean clay. It is more likely that dif fer en tial mu cus se cre tion by the tracemaker en hanced the pos si bil ity for diagenesis. Brom-ley and Asgaard (1975) de scribed a sim i lar ce men ta tion pat tern as de pend ent upon mu cus dis tri bu tion in Pleis to cene spec i mens from Greece. They thor oughly de scribed and in ter preted these back fill struc tures and the bur row ing abil ity of spatangoid echinoids found in situ. The echinoids were de posit-feed ers and formed the back fill by pack ing sed i ment and fae cal ma te rial into menisci and a drain be hind them selves (Bromley and Asgaard, 1975). The drain is in the pres ent case pre served as a thin string.
Fig. 6. Some crus ta cean trace fos sils from the Lilleb³lt Clay For ma tion. A. Ophiomorpha nodosa, side view of ter mi nal end. B. Cross-sec tion. C. Spongeliomorpha isp., lower view. D. Up per view. E. Cross-sec tion.
The ichnogenus Bichordites and its type ichnospecies B. mona-stiriensis were orig i nally coined by Plaziat and Mahmoudi (1988) for the heart-shaped bur row core around a drain. The di ag no sis was emended by Uchman (1995) to con sider the sur round ing meniscate back fill. Later, Demircan and Uchman (2012) mod i fied the di ag no sis for Bichordites to in clude the struc tur ally more com plex B. kuzunensis. The spec i mens from the Lilleb³lt Clay For ma -tion re sem ble B. monastiriensis. Be cause the spec i mens tend to lack the mar ginal rim of the menisci, they are as signed to Bichor-dites isp. A spec i men from the same for ma tion at Trelde N³s (Jutland) was un der the term “hunde-hÝm-hÝm”, il lus trated by Gravesen (1995, p. 18, fig. B). Sim i lar trace fos sils are also known as echinoid bur rows from the Mio cene HagenÝr-BÝrup suc ces sion at Lilleb³lt, prob a bly the Vejle Fjord For ma tion (Radwañski et al., 1975). The mar gin of the Mio cene North Sea had pe ri ods of open and re stricted con di tions with high and low hy dro dy namic en ergy, re spec tively (Radwañski et al., 1975).
Ichnogenus Chondrites von Stern berg, 1833
Chondrites isp.
Fig. 7E
Ma te rial: 3 spec i mens (unit L4).
De scrip tion: Tun nel sys tem ram i fy ing reg u larly down ward to lat -er ally. The an gle of branch ing is com monly acute. The branches are about 2.5–3.3 mm wide and tend to be slightly curved. They are oval in cross-sec tion, rarely cir cu lar, and clus tered into groups. The structureless fill is faintly lighter than the rest of the beige con cre tion. The fill ap pears to be more densely ce mented. Re marks: A com pre hen sive re vi sion of the ichnogenus Chondri-tes was done by Fu (1991). Be sides the mode of branch ing, Uch-man et al. (2012) em pha sized the rel a tive ra tio be tween the width of the bur row sys tem and the width of the tun nels. The ra tio was con sid ered im por tant for dis tin guish ing var i ous ichnospecies. Within the Lilleb³lt Clay For ma tion, the width of the tun nels pre -lim i nar ily sug gests Chondrites targionii (Brongniart). The concre- tionary pres er va tion of the bur row sys tems makes an eval u a tion of the com plete sys tem size dif fi cult. They are there fore de ter mined to un spec i fied ichnospecific level.
Tracemakers of Chondrites have com monly been in ter preted as endobenthic de posit-feed ers (e.g., Simpson, 1957; Osgood, 1970; Bromley and Ekdale, 1984). It is known that Chondrites typ i cally is pro duced in a rel a tively deep tier within the an aer o bic zone (Bromley and Ekdale, 1984). The bot tom wa ter was pos si bly poorly ox y gen ated. The tracemaker could have bene fited from chemosymbiosis, based on hy dro gen sul phide and meth ane (Seila- cher, 1990; Fu, 1991). Al ter na tively, the tracemaker fed from the sur face sed i ment and by its ex cre tory be hav iour filled the bur row sys tem (Kotake, 1992). Be cause a pelletal tex ture is ab sent, this ex pla na tion is less likely in this case.
Ichnogenus Atollites Maas, 1902
Atollites zitteli? Maas, 1902
Fig. 8A
Ma te rial: 1 spec i men (out crop scree).
De scrip tion: Cir cu lar struc ture which is about 75 mm in di am e ter. The thick ness is 15 mm. There is a 2-mm-wide de pres sion in the cen tre. The sur face area around the cen ter is structureless. The area is about 40 mm in di am e ter. The rim is 18 mm wide and par -tic u larly de tailed on one side. It con tains about 22 ra dial lobes. Each lobe is about 7 mm wide. The out er most part of the rim tends to be swol len in thick ness.
Re marks: Only one spec i men was ob tained from a scree sur face.
It is etho logi cally cate gor ised as a fodinichnion. The tracemaker is in ter preted to have mined the subsurface for nu tri ents in a ra dial pat tern out from a cen tral shaft. A sim i lar in ter pre ta tion was made by Durkin (1968) for a sim i lar struc ture with a cy lin dri cal tube, sur rounded by ra dial over lap ping lobes. That struc ture be longs to Gyrophyllites Glocker that was di ag nosed by Strzeboñski and Uchman (2015; see Fu, 1991, Uchman, 1998) as in clud ing ra dial bur rows with subhorizontal, swol len leaf-like lobes that ra di ate from a ver ti cal shaft at one or more lev els. Be cause the RÝsn³s spec i men is char ac ter ised by a structureless cen tral area and en -large ments at the rim, it merely re sem bles Atollites Maas.
A three-di men sional re con struc tion of Atollites italicum Serpagli in di cates that the cen tral cir cu lar area rep re sents a ver ti cal, ax -ial shaft, com pacted dur ing burial (Serpagli, 2005). There are about 30–35 cyl in ders ra di at ing lat er ally from the shaft. Each cyl -in der ends dis tally -in a club. Atollites mi nor Maas and A. zitteli Fig. 7. Trace fos sils from the Lilleb³lt Clay For ma tion. A. Dreginozoum beckumensis, up per view. B. Lower view. C. Cross-sec tion. D. Bichordites isp., meniscate back fill, side view. E. Chondrites isp., cross-sec tion.
Maas are sim i larly char ac ter ised by a cen tral cir cu lar area (Maas, 1902). Cyl in ders are ra di at ing from this area; each cyl in der ends dis tally in a club. Atollites mi nor and A. zitteli dif fer from A. italicum by equally sized clubs in a cer tain spec i men (Serpagli, 2005). The over all size dif fer ence be tween A. mi nor and A. zitteli was em pha sized by Maas (1902). Con sid er ing them as trace fos -sils, size is an in valid ichnotaxobase (e.g., Bertling et al., 2006). Ap par ently, A. mi nor has in ter nally struc tured clubs, pos si bly back filled, whereas those of A. zitteli are structureless (see Maas, 1902, pl. 23, figs 5, 6). The spec i men of the au thors re sem bles the lat ter. Be cause the di ag nos tic cyl in ders are unobservable, prob a bly ow ing to poor pres er va tion, the spec i men is left in open no -men cla ture, Atollites zitteli?.
Ichnogenus Rhizocorallium Zenker, 1836
?Rhizocorallium isp.
Fig. 8B–D
Ma te rial: 4 spec i mens (out crop scree).
De scrip tion: Spreite struc tures of subpar al lel, curved laminae. Di -men sions of in di vid ual lobes are vari able. Pre served in full re lief. Re marks: Only a few spreite struc tures, which are in a poor state of pres er va tion, have been found at the cliff ex po sures in ves ti -gated. They ap pear to have de rived from larger bur row sys tems. For ex am ple, the coil shown in Fig ure 8D is most likely to be from Zoophycos Massalongo. Uchman (1998) de scribed Zoophycos as spreite ar ranged in a helicoidal spi ral and with a cen tral, ver ti cal or mar ginal tun nel. The other spec i mens from the Lilleb³lt Clay For -ma tion may be long to Rhizocorallium Zenker, fol low ing the emended di ag no sis by Knaust (2013). Both Zoophycos and Rhizo-corallium are known to oc cur in the Lilleb³lt Clay For ma tion (Gravesen, 1995). The spreite struc tures were formed by endo-benthos that moved lat er ally back and forth to mine the sub strate for food (fodinichnia). Other in ter pre ta tions may be sug gested (Uchman, 1998, for re view); how ever, the con di tion of the ma te -rial does not al low a fur ther as sess ment.
DIS CUS SION
Pelletal tex ture
Phymatoderma melvillensis from the Lilleb³lt Clay
For ma tion is char ac ter ised by a pelletal tex ture in the bur
-row fill (Fig. 4). This kind of bur -row was made in the clayey
sub strate with a con sis tency like softground. There are no
bioglyphs that could in di cate a firmer sub strate. There are
two ways the bur rows may have been filled with pel lets. It
ap pears most likely that they orig i nated as fae cal pel lets
made by the tracemaker, which was a crus ta cean ac cord ing
to Gravesen (1995). A pelletal bur row fill com monly was
re lated to subsurface de posit feed ing (Fu, 1991), sur face de
-posit feed ing fol lowed by subsurface stow age for later food
uti li sa tion (Miller and Vokes, 1998; Uchman and
GaŸdzi-cki, 2010), or sur face de posit feed ing adapted for ef fec tive
nu tri ent ab sorp tion and subsurface ex cre tion (Izumi, 2012).
An al ter na tive ex pla na tion is that the pel lets hy dro dy nam i
-cally acted as large grains that were trans ported into open
bur rows. This would have re quired deeper ma rine cur rents,
far from the coast line. The au thors con sider this to be less
likely. Sim i lar pel lets were not ob served in the uncemented
sed
i
ment sur
round
ing the trace fos
sils; a pelletal tex
ture
could have been ho mogen ised by com pac tion (Friis, 1995).
Be cause the fos sil ised bur rows are mainly cir cu lar in
cross-sec tion, they must have been ce mented soon af ter they
were aban doned by the tracemakers. The ce men ta tion pre
-vented com pac tion. Diagenetic en hance ment of the bur rows
can be re lated to the pelletal tex ture that fa cil i tated as a con
-duc tor for fluid ir ri ga tion (see Seilacher, 1951). In ad di tion,
the bur rows may have be come ce mented be cause of mu cus
se creted from the tracemakers. The mu cus could have fa cil i
-tated the ce men ta tion. It is well known that mu cus can cause
dif fer en tial pres er va tion of bioturbation struc tures (e.g.,
Bromley and Asgaard, 1975). The out crop qual ity and the
clay-dom i nant suc ces sion do not al low an as sess ment of the
orig i nal depth of bur row ing. Only the low est parts of P.
melvillensis may have been pre served by diagenesis (see
Izumi, 2012, fig. 5; Fig. 4D). Gen er ally, Fu (1991) stated
that P. granulata could have formed up to 50 cm into the
seafloor; with post-com pac tion pres er va tion of 15 cm.
Miller and Vokes (1998) re cog nised Phymatoderma as a shal
-low-tier trace fos sil, with less than 10 cm pre served
post-com pac tion.
Depositional con di tions
The sed i ment of the Lilleb³lt Clay For ma tion is very
clean clay and bar ren with re spect to the re mains of ter res
-trial plants. The lithological units have a re gional con ti nu ity
(Heilmann-Clausen et al., 1985) that makes an embayment
set ting un likely. This in di cates that de po si tion oc curred un
-der very low lev els of hy dro dy namic en ergy, far from the
shore. An ab sence of colo nis ation sur faces in di cates con tin
-u o-us sed i men ta tion. In ter vals of str-uct-ureless clay in di cate
stag nant bot tom con di tions with ox y gen de fi ciency. Al ter
-na tively ex ten sive com pac tion may have erased biogenic
sed i men tary struc tures, in clud ing pelletal tex ture (see Friis,
1995). At least pe ri od i cally, the sea floor be came colo nised
by tracemakers that made the bur row sys tems elab o rated.
The oc cur rence of re cog nis able bur rows may re flect the du
ra tion of colo nis ation win dows and may be re lated to dif fer
-en tial pres er va tion po t-en tial, de p-end -ent upon the chem i cal
thresh old for diagenetic en hance ment. An ox y gen ated wa ter
col umn above the sea bot tom was a pre req ui site for sus tain
-ing macrobenthic life forms. Ir ri ga tion of their bur rows may
lo
cally have af
fected the po
si
tion of the re
dox po
ten
tial
boundary within the sub strate (e.g., Seilacher, 1951, fig. 3;
Bromley, 1996). Heilmann-Clausen et al. (1985) stated that
ox y gen over all was more re stricted in the bot tom wa ter than
dur ing the de po si tion of the un der ly ing RÝsn³s Clay For
-ma tion. There were brief in cur sions of ox y gen.
As ob served in the Lilleb³lt Clay For ma tion,
Ophio-morpha nodosa, SpongelioOphio-morpha isp. and Phymatoderma
melvillensis are char ac ter ised by wall lined mar gin,
scrat-ched mar gin and smooth mar gin, re spec tively. The sin gle
spec i men of O. nodosa is from out crop scree and can not be
linked to a par
tic
u
lar lithological unit. Spongeliomorpha
isp. and P. melvillensis may oc cur to gether in in ter vals of
the clay-dom i nated suc ces sion that are uni form in grain size
and sort ing. It is most likely that the sub strate con sis tency
changed from softground to firmground as the sub strate be
-came firmer over time. In creas ing burial depth re sulted in
dewatering and ini tial com pac tion of the sub strate, while the
Fig. 8. Other trace fos sils from the Lilleb³lt Clay For ma tion. A. Atollites zitteli?, ap par ently up per and lower views. B. ?Rhizoco-rallium isp., sin gle lobe. C. ?Rhizoco?Rhizoco-rallium isp., abraded on the mod ern shore. D. Indet. isp., a cen tral coil of a larger bur row sys tem.
colo nis ation win dow was open. The tran si tion from
soft-ground to firmsoft-ground can re flect a de vel op ment within the
tran
si
tion layer, be
fore it be
came a his
tor
i
cal layer (see
Ekdale et al., 1984). This would ex plain the dif fer ences in
bur row ing be hav iour by the tracemakers. It is note wor thy
that the sub strate was dom i nated by clay and, thus, the sea
bot tom could be ex pected to have been a soupground in the
su per fi cial layer. Be cause of the pelletal tex ture, how ever,
its con sis tency might have been firmer.
The con cept of ar che typ i cal ichnofacies was founded
by Seilacher (1964, 1967). The ma rine ichnofacies re flect
be hav iour con trolled to some ex tent by sub strate con sis
-tency (e.g., Ekdale, 1985; Bromley, 1990). Those formed in
softground are the Psilonichnus, Skolithos, Cruziana,
Zoo-phycos and Nereites ichnofacies. The sea cliff on RÝsn³s
pen in sula has yielded trace fos sils, most of which can be in
-ter preted as fodinichnia that served as a com bi
na
tion of
dwell ing and feed ing lo ca tions. Phymatoderma melvillensis
is the most abun dant fodinichnion. Ophiomorpha nodosa
and Spongeliomorpha isp. ap pear to have had the sole pur
-pose of dwell ing and can be clas si fied as domichnia. The
pres ence of Spongeliomorpha isp. is in dic a tive of bur row
-ing in stiff mud (e.g., Seilacher, 2007, plate 18); it is not
ne-cessarily ev i dence of an ex posed firmground, but of a firm
sub strate at a deep tier. The tracefos sil as sem blage also in
cludes pascichnial struc tures formed by vagile de posit feed
-ers, ex ploit ing the sub strate. Bichordites isp. and
Dregino-zoum beckumensis are ex am ples.
Over all, the li thol ogy re flects pre dict able con di tions,
typ i cal of the Zoophycos ichnofacies (see Seilacher, 1967,
2007; Pem ber ton et al., 1992a, b). The depositional set ting
was clay-dom i nated and char ac ter ised by low
hydrodyna-mic en ergy, with out any ev i dence of storm events. The
bio-turbation took place in a shelf set ting, lo cated far off shore
from sed i men tary sources. The pres ent as sem blage mainly
com prises trace fos sils of mid tiers. The Zoophycos
ichno-fa cies is nor mally char ac ter ised by a low di ver sity of
fo-dinichnia, formed in both mid and deeper tiers. As fur ther
stated by Bromley (1990, p. 241), the deeper tiers may be
ab sent. The pos si ble pe ri ods of ox y gen de fi ciency, sub strate
con sis tency and the dif fer en tial pres er va tion by diagenetic
pro cesses can ex plain the lack of deeper-tier trace fos sils.
Ox y gen de fi ciency may be a ma jor con trol on Zoophycos
ichnofacies, as so ci ated with or ganic mat ter and calm hy dro
-dy namic con di tions (Frey and Seilacher, 1980). How ever,
the trace-fos sil as sem blage of the Lilleb³lt Clay For ma tion
does not rep re sent a typ i cal Zoophycos ichnofacies. The di
-ver sity is mod er ate and trace fos sils, such as Bichordites
isp., Spongeliomorpha isp. and ?Rhizocorallium isp., are
etho logi cally more rep re sen ta tive of the Cruziana
ichnofa-cies. Trace fos sils of vary ing ori en ta tion are a char ac ter is tic
fea ture of the Cruziana ichnofacies, which gen er ally shows
high di
ver
sity and abun
dance of mostly fodinichnia and
pascichnia, formed by vagile de posit feed ers (Pem ber ton et
al., 1992a, b). The ide al ized shoreface model of Pem ber ton
et al. (1992b; see also MacEachern and Pem ber ton, 1992;
Gingras et al., 1998) in di cates that the Cruziana ichnofacies
may oc cur from the lower shoreface, across the off shore
tran si tion and up per off shore, to the lower off shore. These
depositional en vi ron ments are pe ri od i cally af fected by
tempestitic de
po
si
tion, lo
cated above the storm-weather
wave base (e.g., MacEachern and Bur ton, 2000; Seilacher,
2007). The Car bon if er ous off shore de pos its of the Mor row
Sand stone in Kan sas show a low di ver sity of di min u tive
trace fos sils of dis tal Cruziana ichnofacies, re flect ing a sta
-ble de posit-feeder com mu nity (Buatois et al., 2002). In stead
of re flect ing an im pov er ished fauna, traces of deeper tiers
may have oblit er ated those of shal lower tiers (Buatois et al.,
2002; see Bromley, 1990). The Cre ta ceous West gate For
-ma
tion in Al
berta con
tains silt-poor mudstones with a
lowabun dance, dis tal Cruziana ichnofacies and is re cog
-nised as a shelf de posit (MacEachern and Bur ton, 2000).
These show a gen eral lack of lithological con trast be tween
the trace fos sils and the sur round ing sed i ment. Orig i nally
the mudstones were prob a bly thor oughly bioturbated; how
-ever, burial, dewatering and com pac tion may have re moved
most traces (MacEachern and Bur ton, 2000). The Lilleb³lt
Clay For ma tion bears sim i lar i ties to the cases men tioned
above and the au thors there fore ar gue for a dis tal Cruziana
ichnofacies.
CON CLU SIONS
Even though the out crop qual ity is poor, 9 ichnotaxa
have been iden ti fied from the cliff ex po sure of the Lilleb³lt
Clay For ma tion. The tracefos sil as sem blage is rep re sen ta
tive of the dis tal Cruziana ichnofacies. The depositional set
ting was in a siliciclastic shelf en vi ron ment with slow back
-ground sed i men ta tion. The for ma tion is largely struc
ture-less clay, in which trace fos sils are pre served lo cally as
diagenetic con cre tions. Com pac tion of the uncemented clay
might have erased other traces and the pelletal tex
ture.
How ever, the clay orig i nally could have been
unbioturbated, which would clearly in di cate stag nant bot tom con di
-tions with an ox y gen de fi ciency. At least pe ri od i cally, the
ox y gen level was suf fi ciently high to sus tain endobenthic
tracemakers.
Ac knowl edge ments
We are grate ful to the late Kaj Strand Petersen, who en cour -aged us to start this pro ject and gave us ac cess to his un pub lished ma te rial and col lec tions. Maria Gabriela Mángano, Fran cisco J. Rodríguez-Tovar and Al fred Uchman kindly gave con struc tive crit i cal com ments.
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