Taphonomy of Ammonite Condensed Associations - Jurassic Examples from Carbonate Platforms of Iberia SIXTO RAFA EL FERNANDEZ LOPEZ, MARIA HELENA HENRIQUES & LUIS VICTOR DUARTE') Zusammenfassung Abstract . Contents 1. Introduction . 2. 3. Jurassic Exam pies of Condensed Associations from Iberia . Taphonomy of Ammonite Condensed Associations . . . . . . . . . . . . . . • . 3.1. Sedimentary Infilling 3.2. Encrustation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Abrasion and Bioerosion 3.4. Reorientation . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.5. Dispersal . 3.6. Regrouping 3.7. TaphonomicRemoval . 4. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . References .. . . . . . . . . . . . . Taphonomie kondensierter Ammoniten-Assoziationen­ Beispiele von jurassischen Kalkplattformen Iberias Zusammenfassung Spain Jurassic Cephafopods Ammonoidea Condensation Sequence Stratigraphy 423 423 424 424 425 426 426 427 427 428 428 429 429 429 429 Kondensierte Assoziationen van Ammoniten auf epikontinentalen Karbonatplattformen zeigen bei Ablagerung unter Seichtwasserbedin­ gungen verglichen mit Ablagerungen unter Tiefwasserbedingungen ein unterschiedliches Erhaltungsbild. Die Erfassung der Typen konden­ sierter Assoziationen ist bei der Analyse der Beziehung zwischen kondensierten Profilen und stratigraphischen Zyklen van Bedeutung. Assoziationen, die in groBer Tiefe kondensiert sind, sind gute Indikatoren fUr relativen Meeresspiegelanstieg und transgressive Trends. Dagegen sind Seichtwasserassoziationen Anzeiger relativer Meeresspiegelabsenkungen und regressiver Trends. Abstract Condensed associations of ammonites developed in carbonate epicontinental platforms show different preservational features in expan­ ded deposits of shallow environments in relation to condensed deposits of deep environments. Recognizing these types of condensed associations is important when analysing the relationship between condensed sections and stratigraphical cycles. Deep condensed associa­ tions are a very good indicator of relative sea-level rises and transgressive trends. In contrast, shallow condensed associations provide an indicator of relative sea-level falls and regressive trends. *) Authors' addresses: Dr. SIXTO RAFAEL FERNANDEZ-LOPEZ, Departamento de Paleontologfa, Facultad de Ciencias Geol6gicas, 28040 Madrid, Espana. sixto@eucmax.sim.ucm.es. Dr. MARIA HELENA HENRIQUES, Dr. LUls VICTOR DUARTE, Departamente Ciencias da Terra, Universidad de Coimbra, 3049 Coimbra, Portugal. hhenri@cygnus.ci.uc.pt; Iduarte@ci.uc.pt. 1. Introduction In basinal environments, condensed sections are thin stratigraphic units comprising pelagic to hemipelagic de­ posits characterized by very low sedimentation rates. These units are geographically most extensive at the time of maximum regional transgression of the shoreline. Consequently, episodes with very low sedimentation rates are generally related to a relative sea-level rise and abrupt transgression of the shoreline. These condensed sections may be identified in the field on the basis of sed­ imentological evidence. They are associated with marine hiatuses, occurring both as thin but continuous zones of burrowed lithified beds and as marine hard-grounds. They may also be characterized by abundant and diverse planktic and benthic assemblages, authigenic minerals (glauconite, phosphate and siderite) and organic matter (LOUTIT et aI., 1988; SARG, 1988; BOMBARDIERE & GORIN, 1998). In shallow environments of carbonate epicontinental platforms, however, condensed sections are developed at the time of maximum regional regression of the shore­ line. Consequently, condensed sections or episodes with very low sedimentation rates are not diagnostic criteria of relative sea-level rise and transgression in shallow car­ bonate epicontinental platforms. In order to distinguish condensed sections of these two palaeogeographic set­ tings, a clear distinction should be made between rate of sedimentation and rate of sediment accumulation, or be­ tween stratigraphic condensation and sedimentary con­ densation (GOMEZ & FERNANDEZ-LOPEZ, 1994). The rate of sedimentation, or the degree of stratigraphic condensa­ tion, of a stratigraphic interval is calculated by dividing the thickness of sediment by the total time interval includ­ ing the gaps. In contrast, the rate of sediment accumula­ tion, or the degree of sedimentary condensation, of a stra­ tigraphic interval can be estimated by dividing the thick­ ness of sediment by the time interval of positive net sed­ imentation. The distinction between these concepts al­ lows one to predict that the degree of sedimentary and stratigraphic condensation will be higher towards the deep portions of the platforms, whereas the stratigraphic condensation processes without sedimentary condensa­ tion will show the maximum intensity and frequency in the shallowest portions of the platforms. Episodes of relative sea-level rise produce condensed sections composed by condensed deposits. In contrast, episodes of relative sea-level fall produce condensed sections composed by expanded deposits (e.g., tempestites) in shallow carbon­ ate epicontinental platforms. From a palaeontological point of view, very little work has been done on the recognition of the preservational features of condensed associations included in conden­ sed sections from deep to shallow environments. Ammo­ nites have been traditionally used in dating and chrono­ correlation of Mesozoic deposits. However, they can also be used in interpreting some features of the sedimentary palaeoenvironments. In particular, preservational fea­ tures of ammonites can be a useful tool to recognize and distinguish condensed associations formed in separate palaeobathymetric conditions. Several cases of conden­ sed associations of Jurassic ammonites formed in shal­ low environments (FERNANDEZ-LOPEZ, 1997) and in deep environmental conditions (FERNANDEZ-LOPEZ et aI., 1999b) have been recognized in the Iberian Peninsula. The results of previous studies have provided information about the palaeogeographical conditions and the fea- tures of each one of these particular taphofacies. In the present work, the comparative analysis of several ammo­ nite condensed associations developed in carbonate epi­ continental platforms will be shown in order to yield ta­ phonomic criteria to distinguish expanded deposits of shallow environments in relation to condensed deposits of deep environments. 2. Jurassic Examples of Condensed Associations from Iberia The Jurassic examples presented in this paper provide a data set that highlights the differential preservational features of the ammonite condensed associations deve­ loped in shallow environments in relation to deep environ­ ments of carbonate platform: from the Lower/Middle Jurassic transition in the Catalan Basin and from the Low­ er Pliensbachian lumpy limestones in the Lusitanian Basin (Text-Fig. 1). Text-Fig. 1. Iberian Peninsula N 250 km I I Location map of mentioned outcrops in the Iberian Peninsula - Llaberia (LL) in the Catalan Basin and Peniche (PE) in the Lusitanian Basin. The Lower/Middle Jurassic transition in the Catalan Ba­ sin contains many examples of ammonite condensed as­ sociations developed in shallow environments of carbo­ nate epicontinental platform (FERNANDEZ-LOPEZ et aI., 1998, 1999a). The stratigraphic successions of the Low­ er/Middle Jurassic transition observed in two outcrops (Llaberia km 36 and Barranco de Romulla) provide the be­ st examples of this basin of condensed sections devel­ oped in very shallow environmental conditions. These condensed sections span the uppermost portion of the Barahona Formation and the lower portion of the San Blai Formation (Text-Fig. 2). The Toarcian, Aalenian and Low­ er Bajocian are represented by a thickness lower than 50 cm. Deposits of this interval consist of fossiliferous, glauconitic, thin limestones (5-30 cm) interbedded with thinner bioclastic marls, containing ferruginous and pho­ sphatic ooids. Limestone beds comprise wackestone to packstone with recrystallized bioclasts (ammonoids, bi­ valves, equinoderms, brachiopods, belemnites, gastro­ pods, sponges, serpulids, bryozoans, foraminifers, ostra­ cods and algae). Thaiassinoides, Rhizocoraiiium and loophycos are common. Deposits of this facies exhibit high gamma ray counts and a high concentration of ammonites. Strati­ fication surfaces are highly bored and represent time of nondeposition. Common hard-grounds and scarce lateral continuity of the beds in this stratigraphic interval provide indications of both a low stratigraphic completeness and LLABERIA Km 36 250 cm Reelaborated fossils • Resedimented fossils 0 __ ----r------c.. ......... . .......................... . . ... . . . .. . .. . 16 200 -"-':==�--� 14 �=2 .............................................. . .. . 150 --= 100 ItL�C'-J�: .................................... . 50 o LlAS DOGGER Text-Fig. 2. Stratigraphic section of the Lower/Middle Jurassic transition observed in the outcrop of Barranco de Romulla (Llaberia, Catalan Basin). Based on data from FERNANDEZ-LOPEZ et al. (1996). a low microstratigraphic acuity (sensu SCHINDEL, 1982). Chronostratigraphical completeness (i.e., proportion of recorded chronostratigraphical units; cf. SADLER, 1981) is lower to 40 % at zonal scale, although it can reach values of 100 % at stage scale. Stratigraphic success ions in shallow epicontinental platforms are usually more incom­ plete than those formed in deep basins (cf. SCHINDEL, 1982; McKINNEY, 1985; KOWALEWSKI, 1996). In shallow en­ vironments of carbonate epicontinental platforms, however, the fossil record may reach higher continuity than the stratigraphic record. Registratic gaps (i.e., gaps of the fossil record) identified by means of ammonites have generally smaller geochronological amplitude than the contemporary biostratigraphic gaps in shallow epi­ conti nenta I platforms (F ERNAN DEZ -LOPEZ, 1997). Deposits of these condensed sections are interpreted as having been deposited in very shallow environments of open marine platform, as a result of winnowing processes and bypass of fine-grained sediments. The Lower Pliensbachian in the Lusitanian Basin is re­ presented by a thickness of over several tens of metres. The Lower Pliensbachian lumpy limestones is a typical fa­ cies in the Lusitanian Basin (Text-Fig. 3). Deposits of this lithology have been included with the term "Vale das Fontes marls and marly limestones" in the lower portion of the Quiaios Formation (SOARES et aI., 1993; SOARES & DUARTE, 1997). The lithofacies comprises thin limestones (5-40 cm), heavily bioturbated, alternating with thicker, less bioturbated, marly mudstones and bituminous, lami­ nated shales. Limestone beds comprise mudstone to wackestone with recrystallized bioclasts (ammonoids, brachiopods, belemnites, thin shelled gastropods, spicules of sponges, bivalves, radiolaria, ostracods, equi­ noderms and algae). Carbonized wood fragments of cen­ timetric size are also present. Chondrites and other biotur­ bation structures are common. The lumps included in limestone beds and marly intervals are micritic, calca­ reous concretions, subspherical and angular in shape, millimetric or centimetric in size. These facies exhibit high gamma ray counts (PARKINSON, 1996) and a high concen­ tration of ammonites (ELMI et aI., 1988). Since there are no indications of hard-grounds or large variations in sed­ imentation, these lumpy limestones provide both a high stratigraphic completeness and a high microstratigraphic acuity. Chronostratigraphical completeness reaches va­ lues of 100 % at zonal scale. The taphofacies of lumpy limestones and marly intervals with reelaborated ammo­ nites (taphofacies of type 1 in Text-Fig. 3) represents the intervals of deposition when the rates of sedimentation and sediment accumulation reached the lowest values, and starvation reached a maximum (FERNANDEZ-LOPEZ et aI., 1999b). Sediments of this facies are interpreted as having been deposited in deep marine environments, be­ low wave base, induced by sedimentary starving. How­ ever, the presence of reelaborated ammonites implies that some form of current flow or winnowing affected the burial of the concretionary internal moulds. 3. Taphonomy of Ammonite Condensed Associations The examples from the Lower/Middle Jurassic transition in the Catalan Basin and from the Lower Pliensbachian lumpy limestones in the Lusitanian Basin show that low sedimentation rates, high concentrations of ammonites and high gamma ray counts may be associated with the development of condensed sections and ammonite con­ densed associations. These features, however, developed both in shallow carbonate epicontinental platforms during regressions or episodes of relative sea-level fall and in deep carbonate environments during transgressions or episodes of relative sea-level rise. Ammonite condensed associations from shallow environments can show similar preservational features in relation to those developed in deep environments, as a result of the low rate of sedimen- TF1 LS LL TF2 TF3 LITHOLOGY TAPHOFACIES W ° Z � en U:i odZ zO 0° -w !;(....., Z � wt­ (9 en >-w a� w (J) Z w 0::: iiJ co N X Z w � !:Q z z o 0 (J) (J) w w � � --, --, z <>: I () a3 z w ::::; CL 0::: w � -' Text-Fig. 3. Lower Pliensbachian section at Peniche (Lusitanian Basin). Biostratigraphical data are based on ammonites (MOUTERDE, 1955; PHELPS, 1985; DOMMERGUES, 1987; ELMI et aI., 1988; DOMMERGUES et al., 1997). BS � Bituminous shales; HL � Homogeneous limestones; LL � Lumpy limestones; LM � Lumpy, marly intervals; LS � Laminated mudstones; TF1 � Taphofacies of type 1: Lumpy limestones and marly intervals with reelaborated ammonites; TF2 � Taphofacies of type 2: Laminated marls and bituminous shales with resedimented ammonites; TF3 � Taphofa­ cies of type 3: Homogeneous limestones with resedimented ammo­ nites. tation and associated processes such as: high degree of biodegradation-decomposition, synsedimentary mi­ neralization and reelaboration (i.e., exhumation and dis­ placement of the preserved elements, before the final burial; FERNANDEZ -LOPEZ, 1991). During the development of condensed associations both in shallow and in deep environments, biostratinomic processes of biodegrada­ tion-decomposition are generally intense. Ammonite shells commonly lose the soft-parts and the aptychus, as well as the periostracum and connecting rings, before the final burial. Uncompressed, cemented, sedimentary internal moulds of the shell, indicative of low rate of se­ dimentation and early mineralization processes, are abundant. The degree of removal (i.e., the ratio of re­ elaborated and resedimented elements to the whole of recorded elements) and the degree of taphonomic herit­ age (i.e., the ratio of reelaborated elements in the whole assemblage) can reach 100 %. However, ammonite condensed associations of shal­ low environments may show some distinctive preserva­ tional features with respect to those developed in deep environmental conditions, resulting from taphonomic processes such as: sedimentary infilling, encrustation, abrasion and bioerosion, reorientation, dispersal, re­ grouping and removal. 3.1. Sedimentary Infilling In shallow environments, hollow ammonites (i.e., shells showing no sedimentary infill in the phragmocone) are abundant. These hollow ammonites are indicative of very rapid sedimentary infill and high rate of accumula­ tion of sediment, although associated with episodes of low rate of sedimentation. Calcareous, phosphatic and glauconitic, concretionary internal moulds are common, showing evidence of iterative and heterogeneous se­ dimentary infill (FERNANDEZ-LOPEZ, 2000, Fig. 4). Pyritic ammonites are scarce. In deep environments, non-hollow ammonites (i.e., shells showing sedimentary infill in the phragmocone) are abundant. Non-hollow ammonites are indicative of very slow sedimentary infill, and low values in sedimen­ tation and accumulation rates. Concretionary internal moulds are calcareous, showing no evidence of iterative and heterogeneous sedimentary infill. Pyritic internal moulds may locally be common, even as reelaborated elements (Text-Fig. 4F). 3.2. Encrustation In shallow environments, reworked concretions, am­ monite shells an d concretionary internal moulds of am­ monites can be encrusted, developing oncolitic or pisoli­ tic structures. Shells and concretionary internal moulds ' . rv" ,. � "'� i . '\ if, " , .' i " ( ',l .. ',' • , '( I. / f�, "'.lI:' .'t'. ,�� . \ -....... , . Text-Fig. 4. Ammonites (Dayiceras sp.) from Lower Pliensbachian lumpy limestones of the Lusitanian Basin. A-E) Examples of ammonite half-lumps. Specimens are preferentially encrusted by calcareous, stromatolitic laminae on a side. They are reelaborated, calcareous, concretionary internal moulds, maintainin!=) their original volume and form as a result of rapid early cementation. A-C) Specimen BR2; X1, Brenha. D-E) Specimen PE5S/1; X2, Peniche. F) Reelaborated, pyritic internal mould, showing desarticulation surfaces. Specimen PE67/1; X2, Peniche. Call. SRFL (after FERNANDEZ-LOPEZ et aI., 1999b). can present ferruginous and/or phosphatic, stromatolitic laminae, developed during the removal processes. Skele­ tal remains of calcareous, encrusting organisms (such as serpulids, bryozoans or oysters) are very common. Re­ mains of extrathalamous encrusters are developed both on resedimented shells and concretionary internal moulds. In deep environments, reworked concretions, shell frag­ ments and concretionary internal moulds of ammonites can be encrusted, developing oncolitic cryptalgal struc­ tures. Shells and internal moulds can present calcareous, microbial laminae, developed during the removal proces­ ses. Concretionary, internal moulds show commonly cal­ careous microbial or stromatolitic laminae, developed preferentially on the exposed upper side during the exhu­ mation and displacement processes. Ammonite half­ lumps (a particular case of reelaborated ammonites) is a common preservational type (Text- Fig. 4A-E). However, skeletal remains of calcareous, encrusting organisms (such as serpulids, bryozoans or oysters) are very scarce. Remains of intrathalamous or extrathalamous serpulids are only developed on some resedimented shells of am­ monites. 3.3. Abrasion and Bioerosion In shallow environments, signs of abrasion and bioero­ sion on shells and internal moulds of ammonites are very common. Concretionary internal moulds show rounded and bioeroded disarticulation surfaces and fractures. Truncational abrasion facets and roll abrasion facets are common. More seldom, associated with hard and ho­ mogeneous substrates, concretionary internal moulds display ellipsoidal abrasion facets (Text-Fig. 5) and annu­ lar abrasion furrows. Fragmentary internal moulds show high values of roundness and sphericity as well as common biogenic borings. Centimetric borings are common. In deep environments, signs of abrasion and bioerosio­ n on shells and internal moulds are very scarce. Concretio­ nary internal moulds can show disarticulation surface­ s and fractures, but displaying sharp and acute margins. Associated with denudation sedimentary surfaces, intern­ al moulds may show truncational abrasion facets. Frag­ mentary internal moulds can also occur, but bearing no­ signs of rounding or bioerosion. Even millimetric biogenic borings are very scarce. 3.4. Reorientation In shallow environments, ammonite shells and concre­ tionary internal moulds are usually reorientated, with their long axes parallel to the bedding, at firm- and hard­ grounds. Ammonite elements are usually reoriented, even when they are initially included in expanded sediments of channelled facies or deposited by events of turbulence. Ammonites included in tempestites show fining-upwards Text-Fig. 5. Hifdoceras subfevisoni FUCINI. Reelaborated, incomplete phragmocone. The left side has been cleaned (A), carrying off a thin ferruginous crust composed by abundant serpulids. The original ferruginous crust, covering the concretionary internal mould and the ellipsoidal abrasion facet (FE) preferentially developed during reelaboration on the last third ofthe last preserved whorl, can be observed in the right side (B). This specimen was reelaborated from the Serpentinus Chronozone (Toarcian) and abraded in intertidal environmental conditions before their final burial in subtidal sediments corresponding to the Sauzei Chronozone (B