taphonomy of ammonite assemblages from the

Rivista Italianr di P;rleontolo{Ìia e Stratigrrfi,r
volunre 1 l0
r.ro.
I
l
prs.
pp. 239-248
April
2OO4
TAPHONOMY OF AMMONITE ASSEMBLAGES FROM THE MIDDLE-UPPER
OXFORDIAN (TRANSVERSARIUM? - BIFURCATUS ZONES)
IN THE INTERNAL PREBETIC (BETIC CORDILLF.RA, SOUTHERN SPAIN):
TAPHONIC POPULATIONS AND TAPHOFACIES TO SUPPORT
ECO STRATIGRAPHIC INTERPRETATIONS
FEDERICO OLORIZ, MATIAS REOLID 6. FRANCISCO
T.
RODRIGUEZ-TOVAR
Receit'ed October 20, 2002; .lccepte(l October 3, 2003
; Anrnronircs, taphonorrl,, taphofacies, ecostr.rti grlstratiuraphl', Upper Jurlssic, Prebetic, southcrn Sp;rin.
Ke1' tords
ph1,, sequence
Abstract. The taphonomìc anrrlvsis conducted on ,rnrmonoid assembhÍìes h;rs provcn uscful for palaeobiological and p;rl;reoenvironnrcntal reconstructions in upper Middle-to-lower Upper Oxfordian epicontincntal deposits of thc Internal Prebetic (southcrn Spain). T,rphonic
populrtions close to type I indicatc the proximitv, even coinciclence, of
lile lreas for neritic:rrr.rr.nonires (i.e. parautochthonous;ìssen.rblages).
The relationships irÌrons pl'eser\'rrion nroclc, shcll size, r,ithin-bed position, corr;rsion, frlgmcntrrtion, :rnc1 epibionts (c'ncrustrlent includcd),
t,ith lu:.npr-olrcolitic ;rnd condensccl & bioclasts-rich lunrpv-oncolitic
linrestones;rllou.s us to typifv trvo t;rphoi;rcies. Tephofacies I shol's
highc'r r';rlues of nrc,rn shcll size, corrlsìon, cpibionts and encrusrrnenrs,
;rs s'ell ;rs r high nu:lber of spccinrens in quasi-hclrizontll settler.nent.
Thc opposrte chrructcristics serve ro idcntifv Taphofacìes II. T;rphononrìc features, t:rphnflcics and lithofrcies co:rbine in coherence n,ith
progressing third-order tr;rnsgressive-to-highst;rnd svstenr-tract conditions ;rnd the resulting ecostrltieraphic trends, u'hich are registered by
fluctu;rtions in ;rnrnronite irssembllges.
Riltssunto. i-analisi rafononticl condotta sulle associazioni ad
lmnronoidi si è dimostri.Lta urile per le ricostruz-ioni paleobioloeiche
e p;rleoambientali neì depositi epicontinentali riferibili all;r parte alt;r
dell'Oxfordiano medio alla parte bassa dell'Oxfordiano superiore del
Pre'betìco Interno (Spagna merìdionale). Popolazioni tafoniche vicine
al tipo I indìcano la prossin.riti, perfino la coincidenza, delle aree di vira
per tli ;rnrmoniti neritici (cioé :rssociazioni parautoctone). Le relazioni
trl mod;rlità di conservazione, taglia della conchieli;r, posizione nello
strato, corrosione, franrmentazione, ed epibionti (incluse le ìncrostaz-ioni), con calcari grunroso-oncolitici e c,lcari grunroso-oncolitici rìcchi di condens;rti e bioclasti, perrìretrono cli riconoscere due tipologie
di t,rIof,rcies. I-a Tafofacies I mosrra più àlti valori dì taglia nredia della
conchiglia, corrosione, epibionti ed incrostrlzioni, conre pure un elevato nunlcro di esenrpllri in assetto quasi orizzontrle. Le caratterìstiche
opposte servono ad identificare l,r Tafofacies II. I caratteri tlfonomici, lc tafofacies e lc litofacies si conrbinano coerenrernenre con il procedere di condiziorri da trasgressive di terzo ordine;r quelle di sistema
deposizìonale de st,rzionamento alto, e con le risultanti tendenze ecostratisrafiche, che sono registrlre dalle flurtuazioni nelle rssociazioni
ad
lmmoniti.
N
+
Fig.
I
-
Loc;rtion and geolosical sketch
showing the placement of the
sections studied.
llll
s*,.,.nr,r,.0r,,.
-.
-
tnternat Hreoelc
lc
Studied sections
sectron).
Dept. Estr;rtigrafía,v Paleontolosí4, Facultad de Ciencias, Universidld de Grenada. C;rnrpus Fuenrenuevl, l80Zl, Spain.
E-mail: foloriz-(rt ugr.es; fjrtor';rrúr u[ìr.es; mreolìd(r ugr.es
NV (Navalp-
eral section), RS (Rio Segura
F.
240
Oloriz, tr'|. ReoÌid
8x F.
J. Rodriguez-Toúrlr
A
B
Fig.2 - Stratigr:rphic
columns, close views of outcrops, rnd microf.rcies. (A) N;rvalperal section. (B) Rio Segurl section.
crops views only show lumpv-oncolìtìc limestones (I-OL). Bif./Sten. (Bifurcatoides/Stcnocycloìdes Subzone).
Note that out-
Taphonomy of Oxfordian ammonites from Southern Spain
Introduction and geological setting
The Prebetic Zone (Betic Cordillera) shows epicontinental, shelf deposits accumulated on the south-Iberian
palaeomargin during the Mesozoic (Fig. 1). The External and the Internal Prebetic (Jerez-Mir 1973) represent
reiatively proximal and distal areas, respectively, in this
epicontinental environment located in southeastern Iberia. Oxfordian rocks indicate the first hemipelagic-pelagic sedimentation overlying the earlier Jurassic carbonate
shelf (García-Herníndez et al. tlSt;.
In order to identify and quantify taphonomic features in Oxfordian ammonites, and to establish their potential relationships with lithofacies, two sections were
selected in the Internal Prebetic: the Navalperal and Rio
Segura sections (Fig. t). In these sections, mainly Bifurcatus Zone deposits were investigated; these are made
up of lumpy-oncolitic limestone (LOL) and condensed
& bioclasts-rich iumpy-oncolitic limestone (CLOL). In
both of these sections, an uppermost part of the Transversarium Zone could be included at the bottom of the
stratigraphic interval studied, but ammonite biostratigraphy does not provide a higher precision. Ammonite
RE
fr!È
ruouopaor
":#"j:'
Meon
size
Frogmentolion
Inoex
Suosi-horizonlol
posilion
241
biostratigraphy allows the subdivision of the Bifurcatus
Zone rn the Prebetic Zone (Olóriz er al. 1999). In the
studied sections, the lower Bifurcatoides/Stenocycloides
Subzone has been recognized; this is mainly characterized by Dichotomoceras bifwrcatoides Enay and stenocycloides (Ronchadzé), and the upper Grossouvrei Subzone
containing D ichotomoceras grossouorei (Siemiradzkì), bifwrcatus (Quenstedt) and crassws Enay, among other less
known Dicbotomoceras.
Navalperal section
This section is located in the Sierra de Segura (Jaén province)
and crops out across the mountain road between Los Arroyos and the
village of Segura de la Sierra. The section studied is 10./ m thick (Fig.
2A), and mainly shows irregular bedding in nodular-like limestones.
The lower 2 m are dolostones containing scarce and poorly preserved
ammonites and brachiopods (Olóriz et al. 2002b). Above, the secrion
is rnade up of I m thick LOL overlain by 2.2 m of fine-bedded horizons of CLOL rich in ammonites. The uooer 7.3 n.r show the recurrence of LOL.
Rio Segura section
This section is located in the Sierra de Segura (Albacete province), at km 20 on the road between the villages ofYeste and Santiago
de la Espada. The stratigraphic interval studied is 11 m thick (Fig.28),
and bedding in nodular-like limestones
is better expressed than in the Navalperal section. The lowermost 2.8 m are
well-bedded limestone horizons (LOL).
Epibionls
Overlying are 1.6 m composed of ammonite-rich condensed limestones with
a more accentuated nodular appearance
(CLOL). The upper 6.6 m shows the recurrence of limestones with decreasing
nodular-like appearance (LOL).
o
I
/
l
o
Material and methods
In order to analyze the potential
l
o
o
\
/
tm%
0
0
rm*
Inoex
/
I
Frogmentotion
Inoex
Meon
size
these sections, a total of 1240 specinrens .rnd fr.rgments of .rmmonites were
collected through bed-by-bed sampling,
in which the relative abundance of am-
Quosi-horizontol Epibionls
I
(o
)-
!
)
f-sr"
monites, benthic nT acroinvertebrates, and
belemnites was established.
poslllon
I
/
in progress (Olóriz et al. 2002a, c).
In
\
t
Rio Segura sections (Olóriz et al. 2OO2b),
and semi-qurntitatìve chlracteriz.ttions
are
ìm%0
--s"t
Corosion
relerionships between .rmmonite preservation and the type of embedding rocks,
lithology, texture and fabrics have been
carefully srudied in the Navalperal and
7el
1,""",','À
)v
,/
hmonles
t---'.----''-_rm*
Stratigraphic columns showing lithology, raphofacies (TFI and TF-II) and trends in
taphonomic fe,rtures and am-
-^"i,"."."-ht."".
242
F.
Oloriz, M. Reolid
The follorving raphonomic features have been an;rlvzed: a) prcs-
ervrtion rnode; b) shell size; c) n,ithin-bed position; d) corrasion (Brc'tt
& Baird 1986); e) fragrnentation; f) epibionts and encrusrmcnu ancl g)
uncoupling and faceting.
The preservation mode refers to the relative occurrence of inner moulds ivith and/or without neon,orphic shell, and to the tvpè oI
sedinrents that filled up the ammonite carcasses. No quantiiic;ition l'as
.rpplied to this trphononric ferture.
Shell size has been controlled to recognize its potenti;rl relationships rvith the variable completeness of amnronite renrains (Fig. l).
Quantification of shell size allows the identificetion of size intcrvals,
seeking to establish their potential relationships with lithofacies .rncl,
therefore, u,ith stratigraphic intervlls (Fig. a).
Vithin-bed position was taken into account to considcr the rclative significance of quasi-horizontal (parallel to bedding or lc'ss thrn
10"), quasi-vertical (perpendicular to bedding or more rhan tì0"), ;ind
oblique (with inclination beru'een 10" and 80") placenrent of rr.nnrc,nite rcnrains (Fie. 3). Quantification of this trait expresses the nunrber
of ammonite renrains in quasi-horizontal position against other, ;rzinruth;rl positions.
Corrasìon (Brett Et Baird 1986) wrs considered as reverling the
general degradation of the skelet:rl surface due to a variable conlbinrtion of:rbrasion, bioerosion and dissolution (Pls. 1-2). This taphonouric
ferture u,iìs quantified (Fig.3), according ro rhe Corrasion indcx (Ci)
proposed bv Olóriz et al. (2002b). According to these îurhors, rhrec
grades of corrasion r.ere established: low (LCD), mediunr (MCD) rnd
high (HCD) for less than 1A"/", l1"k to 60% and higher than 60'2, of
worn material. Ci values rvere obtained fronr the equation belou,, u,here
N is the total number of specimens, and n the number of spccimcns
k
F.
J. Rodrigucz-Tocar
callv articulatecl structures. To quantif,v frrgnrentation, the Fraqmentation
index (Fi) ri,as.rpplied to inform ;rbout the degree of fr,rgnrentation (Fig.
3) among a set of individuals (Olóriz et al. 2002b). According to these
authors, los, (LFD), nrediun.r (MFD) rnd hirh (HFD) degrees of fragnrent:rtion g,erc idcntified: "LFD for fr:rgnrentltion th;Ìt does not affect
sienific;intlv the shape and size of the specinrens (rnrmonite remrins are
essentialh' conrpletc; HFD for fragrlentation th:rt affects shape and siz-e
signifìcantll' (renrlins lre little representiìtive of original dinrensions and
shape; and MFD for fr;rgmentation bet*'een tlre nvo clegre.es previouslv
considered (HIrD and I-l-D)". Fi values were obtrined from the equation
belos', whcre N is thc tolrl number of specinrens, and n the number of
specimens u
L'-
ith HFD, MFD and LFD, respectively.
[(nHFDx 100) + (nMFDx50) + (nl-IrDs l)]
N
Epibionts rnd encrustnrent havc been analvzecl paving special
attention to the potcntial difference betu,een the upper and the loq,cr
side of the anrnronites, in that concernine the type of organisms and
densitv of coloniz;rtion (Pl. 2). Qualitative quantification w:rs brsed on
naked eve;rnd n-ricroscopic obsen'ations conductcd on selected samples (Fig.3, Pl.2).
Uncouplinu and faceting were also analyzed, thc forn-rer point-
ìnr to disgregation of inner moulds follorving sepral surfaces and lines
of u'horl overlappinu (Olóriz et al. 2000). To confirn uncoupling as a
t;rphononric fcarure, but not an aftermost erosional one, the evidence
for this trait rvls accepted fronr u,ithin-bed specinrens onlr,. The occurrence of facets and their relatìve oosition was analvz-ed.
rvith HCD, MCD and LCD, respectively.
[(nHCDx100) + (nMCDxsO) + (nLCDx I )]
N
Frrgrnentation was considered as rhe resuh of breaking but not
disarticuletion, the latter being interpreted as the separation of biologi-
ì
00%
Results
The nodular-like appearance of Bifurcatus Zone
deposits in the Navalperal and Rio Segura sections is not
related with nodules originated during early diagenesis as
was the nodularity in typical ammonitico rosso and related
facies (see Farinacci & Elmi 1981, for a wide-ranging review of this subject, and updated interpretations in Clari et
al. 1984; Clari & Martire 1996;Marrire 1996).ln conrrasr,
microfacies of wackestones and packstones rich in lumps,
oncoids, intraclasts, aggregate grains, and peloids are typical (García-H ernandez et al. 1981; Olóriz er al. 2002a, b, c;
Fig.2). Grain-supported fabrics are dominant. The moderate abundance in planktonic forams (Globuligerinidae)
is characteristic, as is their peak in reiative abundance in
of the Bifurcatu s Zone (mean value close to
45%). Lumpy-oncolitic limestone (LOL) can be considered the background facies, showing variability in nodular-like appearance, and recurrence in the Bifurcatus Zone.
The proportion of ammonites per rock volume is usually
low. Fine bedded, condensed and bioclasts-rich lumpy-oncolitic limestones (CLOL) are typical of a condensed interval showing a higher proportion of ammonites per rock
volume. In these horizons, packestones dominate.
In the sections studied, ammonites are 75o/o, benthic macroinvertebrares 21"h, and 47n belemnites. Major
fluctuations in the composition of selected ammonite
a lower part
EEEEE
EgEEE
O;OOO
CDYO.NN
tr-lO;O
.oóA
o.
Fig.
+ - Size-cun,es
for amnronite re'mains in lurnpv-oncolitic limestone (LOL) and condensed 6i biochst-rich lumpv-oncolitìc
linrestone (CLOL).
groups are shown in Fig. 3. Among ammonites, perisphinctids are 460/o, phylloceratids 29"1, (Sowerbyceras
19"/"), haploceratids 1,7"/", and lytoceratids 1o/". The remaining specimens are indeterminate. Usually, the recog-
Tapbonontl, of Oxfordian anntonites
nition of suture lines is difficult, and aptychi are extremely
rare. It is worth mentionins that the majority of the an-rmonites with identifiable suture lines are phraemocones,
body chamber preservation beins scarce and selectively
more frequent in haploceratids (e.g. Ocbetoceras (Pl. 1 DE), Trimarginites, Glochiceras (Pl. 1 B-C)). Macroconchiate specimens are rare and represented by inner to middle whorls (e.g. Passendorferia (Pl.1I), Subdiscosphinctes,
Euaspidoceras (Pl. 1A)), but incon-rplete phragmocones
of great size ammonites exist (e.e.. Ewaspidoceras).
Ammonites show dominant preservation as inner
a variable presence of neornorphic, calcitic
shell, especially in the flanks placed downwards, but exceptions occurs (Pl. 2). No significant difference has been
identified between the composition of the shell infilling
and that of the embedding sediment other than those related to infilling processes.
Averase shell size from the two sections studied
moulds, with
shows small vaiues (Fig. 3), and minor variations between
these sections (33 mm in Navalperal against 27 mln in Rio
Segura section) are present. On the whole, shell size below 30 mm is 537o in the Navalperal section and 66% in
Rio Segura section, and these values are obtained irrespective of the taxonomic group registered. However, a difference in the average shell size has been found between
CLOL and LOL lithofacies (Fig. 4), the latter showing
lower values (40 mm in CLOL vs. 25 mnr in LOL). Fluctuation in average shell size was found throughout the
studied stratieraphic interval in the Navalperal and the
Rio Segura sections (Fig. 3).
Within-bed position largely depends on amnronite
size, or on the size of ammonite remains. The greater the
fossils the more frequent their quasi-horizontal position
(registered in CLOL) (Fig. 3). Smaller ones offer a more
variable orientation, which probably indicates a limited
incidence of burrowing in mixing within the sedimentwater inter-phase, together with a major probability for
oblique settlement in smaller and/ or more globose shells.
Fluctuation in within-bed position was found throughout the studied stratigraphic interval in the Navalperrrl
and the Rio Segura sections (Fig. 3).
Corrasion is well registered, but its intensity is variable as demonstrated by Ci values (Fig. 3). Higher values
have been registered from CLOL and in specimens of
greater size, which are more abundant in this lithofacies.
Fluctuation in Ci values was found throughout the studied stratigraphic interval in the Navalperal and the Rio
Segura sections (Fig. 3).
Fragmentation is high on the basis of the fact that
a majority of ammonite remains showing suture lines rrre
phragmocones. Concerning body chamber preservation,
this is more common, even typical, in haploceratids (e.g.
Ocbetoceras, Trimarginites, G lochiceras),an.ìong which cases
of preservation of lappets are known (e.g. Glocbiceras) (Pl.
1B). However, adapertural (i.e. peristomal) structures are
unknown from perisphinctids and other ammonites. On
fron
Southem Spain
243
the rvhole, the thicker deposition and a relative hiqher fragmentation correlate in LOL. Fluctuation in Fi values was
found throushout the studied stratigraphic interval in the
Navalperal trnd the Rio Secura sections (Fig. 3).
Epibionts ,rnd encrustment were selectively reeistered in the upper and lower sides of ammonite remains,
especially in evolute shells of sreater size. The nrost significant difference rvas found in the composition of the
colonizing biota. On the upper, exposed surfaces, epibionts are dor.ninated by nubeculirrnds, Tubiphyfes, and
benthic microbi:rl con.rmunities, the growth of the latter
controlling the sedin.rentary coverine of the umbilicus
(Pl. 2C, D). On the lower, protected surfaces, serpulids,
Terebella, irnd bryozoa are characteristic, but the latter
are occasion,rlly known fror-n exposed surfaces (Pl. 2AD). Ubiquitous colonizers (i.e., with no preference for a
given surface) are siliceous encrusting forams and Bwllopora. Fluuuation in the relative abundance of epibionts
was found throughout the studied stratigraphic interval
in the Navalperal and the Rio Segura sections (Fig. 3).
Uncoupline and facetine (anchor facets) are rather
rare features, and rn,rinly identified in perisphinctids from
condensed intervals (CLOL). The precise identification
of facetine is frequently limited by the corr-rbination of
corrasion and encrustment.
Interpretation
Lithology, texture trnd bedding in CLOL compared
to LOL, which show no difference in the dominant fabrics, support the interpretation of the former as condensed deposits. However, no taphonomic condensation
(Gómez 8r Ferníndez-López 1994) has been proven in
CLOL from ammonite biostratieraphy, except the possibility for a minor one related to maximum condensation close to the boundarir between the Bifurc:rtoides/
Stenocycloides Subzone and the Grossouvrei Subzone
(microstratigraphic disorder resulting in :r shared horizon for the last occurrence of Dichototttoceras falculae and
the first one for D. bifurcatus?). In addition, the stratigraphic range of LOL reinforces its interpret:rtion as representins background depositional conditions in the area.
At first, sr.r.raller average values of ammonite remains
in LOL, and their higher sirnificance, corresponds with the
record in the Rio Segura section, u,hich represents areas in
the outer inner to nriddle shelf. The smaller shell size registered fron.r LOL (Fig. 4), toeether with higher frasmentation, accords n,ith a potentially higher incidence of transport for amrnonite carcalsses. Higher dominance of ammonites showine within-bed position parallel to bedding in
CLOL should result, mainly, fror.r.r lower enersy conditions
and less post-mortem transport during ,rn early part of
the Bifurcatus Chron, when the sedinrentation rates were
lower (Fig. 3). This interpretation accords with long lastin* exposure of skeletals and inner n.roulds in CLOL, and
F.
244
Oloriz,
.ll.
Rcolid
&
thc registered increase of corrrlsion. The rclativelv lou'er
frasmentation found in CLOL is ,'rlso cohercnt rr'ith lou'cr
energy during lou,er sedintentation rates thit chirracterized
maxinlrl condens:rtion within an earlv p,rrt of the Bifr'rrc:rtus Chron (Fiff. 3). Dr,rrins thlt tinrc, frequent coloniz-iltion
of skeletals ,rnd inner r.noulds bv epibionts (cncrttsttllent
included) supports the interprctation of
mentxry pauscs (hiatuses) 's'ith exPosition of trtutt.tot.titc
renrains on thc sea-floor, as rr'ell i1s the:lsstlrllption of se-
freclucnt scdi-
lectivitl. in coloniz-er epibionts (diffcrence irr rcl;rtive photo-dependencc). Thc rrrc occllrrencc of uttc.>tlpling and
faceting agrees with the occurrcnce of cpisodes oi highcr
energy affccti n g horiz-ons relirtivell' r-rncon sol idrltcd u'ithin the sedinrcnt-wate r inter-phase in thc se,r-bottonr. The
lou,er rate of sedir.rrentettion relirted to CLOL deposition
rvould favor the incidcnce and rccord of cpisodic Proct'sscs
resulting in uncor'rpling and f,rceting. Hcavv corrtrsion lnd
encrustnrcnt nrake the l,ttter occilsion,rllv idcntiii,rblc onlv
in axial sections unde r microscope.
Taken into account the composition ,lnd gcner;ll
taphonon.ric tr:rits
tered,
of thc arttrttonite
arssenrblirges regis-
it is possible to recosniz-e taphor.ric
poptrl;rtions
close to thc n'pe 1 dcfincd bv Fcrníndez-l'-ópcz (1995)'
which shon's ,r high ratio of microconchiates to rll;ìcroconchiirtes. Horvcvcr, the stiìte of prcservt-ttion u'ith ,r usually moder:rtc to high degree of encrustnrcnt irllpe dcs thc
precise recognitiorr of suture lines ,rnd' thelefore' that oi
juveniles. In addition, it is difficult to cstablish if fr,rgnrer.rtation occurred r-rtainlv oll the wi'Ìter coltttlrn or on thc sert-
I /. Rorlr'iguez-Tit"'ar
conditions, u,hich accords s'ith the str:rtigrarphic range
of LOL (Fiu. 3).
The increlse in shell size, corrasion' encrustments
,rnd epibionts iront the Transversariunl? Zone ro the lorver part of the Bifurcattus Zone (Fig. 3) correlates rvith the
,urnot'". of taphofacies II by taphof,rcies I, arnd that of
lithofacies LOL by CLOL. It is in accordilnce with irn incrcased exposure tinte of antrrlonoid clrc:lsses on the sea
floor, u,hich rvas favored b.v,r decreased sedinlentltion rrte'
The taphonornic trend described abovc is associlttecl rvith the increase in anrnlonoids' against other macro-r.rregainvertebrates, and irqrees with deposition during
developing transsressive-systelll-tr:lct conditions (Olóriz
et,rl. 2002r, b, c). The reversal of the t,rphonornic trend
described was found in thc upper part of the Bifurcatus
Zone. Thc new trend correlates rvith the recurrencc oi
taphof,rcies II and lithof:rcies LOL, and it is interprcted
as resulting fror.n a higher sedin.rentation rater togethcr
rvith the conconrit:rnt diminishing of the exposure tinle
of irt.nrrronoid carcasses in ir higher energy environnrent'
Throughout the upper p,rrt of the Bifurcatus Zone, the
dinrinishing of ,rntr.uonoid record accords with deposition during developin g hiuhstand-system-tract cor.rditions
(Olóriz- et al.2OO2a, b, c)' On the basis of the above, the
ch,rngcs in the corrtposition of itt.ttnronite asserllblages depicted in fig. 3 arc interpreted in tert'us of ecostratigraphic
trends (Olóriz et rl. 1995) that correspond to the cnlargenrcnt and then restriction of atnnlonitc ccospltcc on the
Prcbetic Shelf durins thc Bifurcatus Chron.
bed. but their contbinirtion can be assunred. No sisr.ìificl.tnt
difference in the tvpe of taphonic population h,rs bcer.r
,r probablv
lower incidcnce ol' post-tììortelìl trensport in ,rnrtuonite
rssemblagcs girthered frorrt the condensed irrte rl'al relrrted
to CLOL. A ntixttrrc of floated spccinrcns lnd relirtivelv
autochthonous sunk oncs (Pl. l; differenti,rl sinking relatc'd, :rt least, to shell r.t.rorpl.rolosr' ,ìnd sizc; Chln.rberlain
et al. 19811Olóriz et al. 1996; for,rn extended, complenlent:1rv treatrrlent see Macdir Er Scil:rcher 1996), is e nvisaged :rs ,r valid picture of the anlt.tlonitc assel.rlbl,rges
recovered, rvhich are interpretcd trs p:rrautochthonous'
On tl.re basis of thc taphononric ,rnd lithologic features, t,rphofircies I and II have bcen rc'cognizcd ar.rd used
to complenrcnt the interpretxt;on tlf ch,rnges in dcpositionirl conditions (Figs. 3, 5). Tephofacies I' relrted to
CLOL, shorvs the contbination of highcr r':rlucs in qtr,rsihorizont,rl position of antt.ttonitcs rclllilins. colonizatiorl
by epibionts, averase shcll size atrd corr,rsit.,n, the vrrlues
of the lltter tu'o being speci,rllv higher thiln in tirphofircies
II (Irigs. 3, 5). In contrtst, lon'er frasmentation is tvpical
of trphofacies I registered in CLOL. According to their
respectivc strirtigrrphic rirn!ìcs (Figs. 3, 5), taphofircies
I is considcrcd to be rchted to lor'ver seditrlent;rrv rittes
and, thcrefore, it is rcstricted to the conde nsed interr'tll
composed of CLOL. On thc other hand' tlphofircies II
is considered to represer.rt br'rckground environr.llentarl
recognized in the Bifurcittus Zone, othcr thrln
PLATE
I
front thc BiîurcrÌtus Zonc' Notc lrodv chilnlbcr
prescrvirtion (u,hite triangles) ;rnd peristonlll structures (lappcts stlnrì
out) in h;rploccratids rg;tinst llrlrinlv phllilnrocone Pr!'serviltton (no
nr,rrks) in evolutc shclls (euaspidocer.rticls, peltoccr.rtids' perisphincticls). Irregul.rr upPer surfiìce (probablv due to thc corrrbitrrrtion of
i.rccting rncl corrasion (blrrck trianglcs) is usu;rl. (A) Right-siclc vierv of
Eu,rspìdoceras sp. shos'ing the withirr-becl los e'r surfrrce of incomplete
phr.rsrnoconc. NV-Z- l: (ll-C) Right- and left-side vicu s of G/oclrtoirs
gros'n bodl'
cÌ. subcl,rusurtr (Oppcl) shos'ing a prob.rblv incorlple te lv
sp;ltusnrall
rtncl
l.lppct
l,rterll
ch,rnrber with thc clongi.ltc ilrnì o[ thc
lltc cnd (the ventlal I.rppet is ricll prescrvcd: B), rrnd fr;rgnlc'ntecl bodv
Sclected lr.ttt.uonitcs
ch;rnrbt't's'ith partiel prescrr';rtiot.t of .ldorrrllv concilve Vclìtro-llterirl
.rrch (C), both in thc sithin-bed loser suri.rces' NV-8-5'+' NV-8-55;
(D-E) \'cntral and right-side vie rls oI OcDerrrcuas (Ochctocaras) cdnalitul,ttutr (r: Buch) shos'ing boclv chlnrbcr prcsen'ltion in s'ithin-bed
Iol'cr surilcc, NV- l+--1 l; (li) l-eft-sidc I'ics' of .ltrrrt'Dirri'r sp shou ing
inconrplete phr,rgnrocon.' in sithin-bcd los'er surf;rce' N\'-9-R; (GH) \tntr.rl ;rnd cft-sidc vies's of l;rrge spt'cinlcn <t[ Subdidxúontoccras
bifttrcatus (Quenstedt) shon'inìÌ irrcgul,rr uPpcr surfilce (right G) and
bcttcr pt'cscn ccl rvithin-bcd lorver surf,rce .t'ith crotvcled ribbing in inf
conrpletc ltoch' ch.rnrber
(H), NV-ó-61 (l) Right-sidc vict oI
nucletts
sp. rcsenlbling nricroconchiiìte .ldults ("Ettaf itei')
shos ins the within-becl los'e r surfrlcei RS-(r-6.
oi lùssurdolanr
Ltpl.rottottt) ol OrfìtrLtt.'tt,rttttttrttttt's lrottr.\otrtltt'nt .\lrtìtt
I r,!
.ò,,(z
!r/
1
,--
3
r)
,J
tj-...i.,
t'-'
2
.4,
i.(
'-'
..
i
"' t-'l-+
.
.,
-,
..i'
B
I
7
D
10
È loi
12
Aa'
43
4-o)
àGr
a |]ib-''€P.
Ly'
f.
\*_
ar# \
11-lf
l3
F
.7
....,-..--.-
-.
5-i14
, -,{.1
Ysl
'
ì
f,l "
,'{.fr
H
PI-ATL,
2
siclc ;rnd
Dottedlineitlr.thcpcriphel..'nîinncr
boththetrpper.rncll<l*'.ir.sides,btttprcserr';ltitlnofscpt.rininncr|oc.rtions[3.Note
brcakirrgbèioreofthcgener.llizeclt1i.ssolutionoftlrt.otrtcrshc|||2];lrrddispl;rcenlentofshelldebris[9],
and nrrisivc BMC grorvth l6-71 uith rrepperl snr,rll Iunrps [8], .rnd cpisodicrl exposure of hard surfaces evidenced bv coloniz-ation of brvozoa !]
,rncl scrpulids [+] .iÀi cduc cont,rct nith ri'"1.,.1r is]r Nú-si:ó. (C-D) Inner nroúld,"ith targely unpreserved epigenized outer shell in boíh the uppcr lnd lo,uer
[.{]. ancl rcvcrsccl geopet.rl infilling
nrould shou,in g prescrviìt iorl
lrrnrinrtecl BMa t7l
nitc she ll rnd juxt,rposerl clictvicl spongc [15] .rnd br.rchioprocl [5] rcnrains, ;rnrl sections of srr.rlll tubes of ;rnnelids
!+l;
NV-+-47.
Taphononl, of Oxfordian antmonites from Soutbern Spain
LOL
x9FÈ__c
9
iF
ts.e ua .ìY
o
:=
;H
cLoL
(TF-rr)
È
xx
H
Ògqqtr
=c99F
xo9-^tr
.!x
!
P
'òX
- H
ó6
[TF-r)
p; -o 3: =p
o
;
a-
ó6
À
,i
K
Averaged v,rlues of taphonorrric features in lumpy-oncolitrc
limesrone (I-OL) and condc.nsed & bioclasts-rich lunrpvoncolitic lìnrestone (CLOL) uscd to identify taphofacies I
and II (see text).
247
(LOL) and condensed & bioclasts-rich lumpy-oncolitic
limestones (CLOL), serves to complement palaeobiological and palaeoenvironmental reconstructions in upper
Middle-to-lower Upper Oxfordian epicontinental deposits of the Internal Prebetic (southern Spain).
The combination of quantification, as well as the
semiquantitative-to-qualitative evaluation, of taphonomic
features, and the analysis of their relationships with lithofacies, supports the characterization of taphonic populations and taphofacies. Taphonic populations close to type
I indicate the proximity, even coincidence, of life areas for
ammonites (i.e. rather parautochthonous assemblages).
Two taphofacies have been distinguished: Taphofacies I
shows hicher values of mean shell size, corrasion, epibionts and encrustments, as well as a high number of specimens in quasi-horizontal settlement. The opposite characteristics serve to identify Taphofacies II.
Trends in taphonomic features are strongly related to changes in lithofacies and taphofacies, and can be
interpreted in terms of systems tracts evolution during
third-order tectono-eustatic cycles and their associated
ecostratigraphic trends for ammonites.
Achnouledgemenr. This research was financed by project BTE-
Conclusions
The taphonomic analysis conducted on ammonoid
from lunrpv-oncolitic limestones
assemblages, gathered
3029 MCyT, Spain, within the frame of the EMMI Group (RNM178,
Junta de Andalucía). The authors would also like to thank the reviewers P Monlco and E. Cariou for their critical reviews and suggestions
that inrproved the manuscript.
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