Sixto R. Fernández-López

Condensation processes in shallow platforms

Abstract Sequence stratigraphy allows elucidation of the time-space relationships of deposits in sedimentary basins. However, the relative distribution of unconformities and condensed sections can neither be used as diagnostic criteria for particular palaeogeographic settings, nor as indicators of maximum regional transgressions. Condensation processes also occur in the shallowest portions of platforms and during maximum regression episodes. Three separate processes of condensation are here distinguished: stratigraphic, sedimentary and taphonomic. These three categories of condensation can be used to analyze patterns of sediment distribution, and also to estimate lateral and vertical variations in sedimentation rates. Contrary to the usual interpretation, the maximum values of rate of sedimentation and degree of accommodation of sediments, as well as the highest relative sea level, occur in the lowest levels of the shallowing-upward sequences. The distinction between these condensation processes allows one to predict that the degree of sedimentary and stratigraphic condensation will be higher towards the distal portions of the platforms, whereas the stratigraphic condensation processes without sedimentary condensation will show the maximum intensity and frequency in the shallowest portions of the platforms.

Ammonite succession at the Bajocian/Bathonian boundary in the Cabo Mondego region (Portugal)

The Cabo Mondego outcrops exposed along the cliffs, on the western margin of the Iberian Plate, show an expanded stratigraphic section of Lower Bathonian deposits containing abundant ammonoids. Upper Bajocian deposits correspond to similar facies, of muddy limestones alternating with marlstones, although ammonoids are scarce. A detailed succession of ammonites across the Bajocian/Bathonian boundary has been recognized at Cabo Mondego, which can form a useful bio- and chronostratigraphic standard for the Lusitanian Basin. The revision of previous collections from the classical section and new field samplings of two other separate sections allow the recognition through up to twenty metres of thickness, the highest zone of Bajocian (Parkinsoni Zone) and the lowest zone of Bathonian (Zigzag Zone). The Parkinsoni and the Zigzag zones established for NW European areas and belonging to the Northwest European Province, can be identified in the Lusitanian Basin, although the ammonite fossil assemblages are composed of Submediterranean taxa. However, a subdivision of the Parkinsoni Zone is not possible, due to the scarcity of well preserved ammonoids. The Zigzag Zone can be recognized and characterized as composed of two subunits (Parvum and Macrescens subzones) as represented in diverse European basins of the Submediterranean Province. Ammonite fossil assemblages of the Parvum Subzone may be grouped into two successive horizons, which are biochronostratigraphically equivalent to the subdivisions of the Convergens Subzone distinguished in the Digne-Barreme area (SE France). New biochronostratigraphic data on the Bigotitinae, youngest members of Leptosphinctinae and oldest members of Zigzagiceratinae are relevant in understanding the evolution and faunal turnover of the West Tethyan Perisphinctidae during earliest Bathonian. The ammonite succession at the Bajocian/Bathonian boundary in the Cabo Mondego region (Portugal) represents one of the most complete biostratigraphic records so far recognized on the Iberian Plate.

NEW EARLY BATHONIAN BIGOTITINAE AND ZIGZAGICERATINAE (AMMONOIDEA, MIDDLE JURASSIC)

Several tens of Lower Bathonian Bigotites from Digne-Castellane region (SE France) and Cabo Mondego area (Portugal) have been reviewed. Three species have been distinguished in the lowermost subzone of the Zigzag Zone (Parvum Subzone) just above the boundary Bajocian to Bathonian: B. diniensis Sturani [M+m], B. sturanii sp. nov. [M+m] and B. mondegoensis sp. nov. [M+m]. In the Bas Auran area, a chronocline from evolute, strongly ribbed and constricted forms (including B. sturanii and B. diniensis ) to involute forms with blunt, moderately prominent ribbing and weak constrictions (including B. mondegoensis ) can be recognized. The shared taxa B. mondegoensis sp. nov. and possibly B. diniensis Sturani permit detailed subdivision and correlation to be established between ammonite fossil assemblages of Parvum Subzone in the Lusitanian and Alpine basins. A separate genus of Zigzagiceratinae, Protozigzagiceras g. nov., is proposed to encompass P. torrensi (Sturani) as type species. These new palaeontological data about the youngest members of Bigotitinae and the oldest members of Zigzagiceratinae are of biochronostratigraphic importance for the subdivision and correlation of the basal Bathonian Zigzag Zone. Three successive biohorizons can be identified at the Parvum Subzone in Bas Auran (French Alpine Basin) and Cabo Mondego (Lusitanian Basin): Diniensis, Mondegoensis and Protozigzagiceras biohorizons.

FOSSILIZATION OF AMMONITES AND SEDIMENTARY EVENTS IN DEEP ENVIRONMENTS OF CARBONATE PLATFORM (HIGHEST MIDDLE TO LOWEST UPPER OXFORDIAN, IBERIAN RANGE, SPAIN)

In the Ricla area (Zaragoza, Aragonese Branch of the Iberian Range), at the top of the Yatova Formation, grey-reddish wackestone limestones grade into yellow-green siliciclastic limestones of the Aldealpozo Formation. These changes of facies between the two successive formations are associated with syndepositional palaeoreliefs developed during the Late Oxfordian. The uppermost deposits of the Yatova Formation represent an Oxfordian condensed section, from the upper Bifurcatus Zone (Middle Oxfordian) and Hypselum Zone (Upper Oxfordian). These deposits are interpreted as developed in an open marine, moderately deep carbonate platform, showing uniform low-energy conditions with extremely reduced carbonate and terrigenous background sedimentation, and very low sedimentation rates. The low diversity of the benthic fauna, scarce development of sponge bioherms and ammonite populations inhabiting the platform are palaeobiological criteria which corroborate these palaeoenvironmental conditions. Ammonite assemblages are composed of Sub-Mediterranean taxa. Over 900 ammonite specimens have been collected from the upper Bifurcatus and Hypselum zones. Oppeliidae (45,2%) and Perisphinctidae (37,9 %) are dominant. Aspidoceratidae (14,3%) are common. Haploceratidae (2,2%) are scarce. Two phylloceratids and a lytoceratid have been found. Ammonoids are commonly preserved as concretionary calcareous internal moulds of reelaborated elements. Resedimented shells are scarce. The degree of packing of ammonite remains and the stratigraphical persistence display high values. Taphonomic features indicative of sedimentary starving in deep carbonate platform environments are: 1) high concentrations of reelaborated ammonites, 2) taphonic population of type two, 3) phragmocones completely filled with sediment, and 4) homogeneous concretionary internal moulds, bearing no signs of abrasion, bioerosion or dense encrusting by organisms (such as serpulids, bryozoans or oysters). In conclusion, the occurrence of these ammonite associations confirms the development of an advanced deepening phase, within a 3rd order deepening/shallowing cycle, in the Aragonese platform, during the late Bifurcatus to Hypselum zones.

THE MIDDLE JURASSIC EASTERN MARGIN OF THE IBERIAN PLATFORM SYSTEM (EASTERN SPAIN). PALAEOGEOGRAPHY AND BIODISPERSAL ROUTES OF AMMONOIDS

From a sedimentological and palaeogeographical point of view, in the eastern margin of the Iberian platform system, Middle Jurassic deposits are represented by a thick pile of carbonates with minor marls interbedded. Different facies, within the Tortosa, Aragonese and Castilian platforms, pertain to two categories: internal platform and external platform facies. A system of extensional faults was the major factor determining the differentiation of several shallow, carbonate, epicontinental platforms. The so-called Iberian Basin was a shallow intracratonic platform system, in which a relatively deep extensional basin (the Catalan Basin) started to be differentiated during the late Bajocian. In the eastern part of this faulted platform system, the occurrence of a listric fault, dipping towards the east, conditioned the development of a late Bajocian differentiated subsident area named the Tortosa Platform. From a palaeobiogeographical point of view, the ammonite fossil assemblages of the Iberian platforms are composed of Sub-Mediterranean taxa. At the eastern margin of the Iberian platform system, the appearance and distribution of shells of ammonoids were taphonomically and ecologically controlled by regional changes of relative sea level. A Sub-Mediterranean zonation can be recognized, although most of the ammonites shells correspond to adult individuals arrived by necroplanktic drift from more open-marine or oceanic areas. The following criteria are relevant in recognizing the maximum deepening episodes in the Iberian platform system: colonization (including ontogenic development) of open-marine taxa, immigration of oceanic taxa and development of endemic species. In terms of Sequence Stratigraphy, the development of expanded, external platform sections in the Tortosa Platform during the Garantiana Biochron (late Bajocian) is noteworthy. During this episode, the Iberian platform system reached the maximum bathymetric values and acted as a biogeographical dispersal area for some taxonomic groups of ammonoids between the Western Tethys and the Proto-Atlantic.

Taphonomic Analysis of the Lower Pleistocene Pirro Nord Fossil Locality (Pirro 10 Site, Puglia, Southern Italy): A Depositional Model for Vertebrate Assemblages in a Karstic Environment

ABSTRACT Taphonomic analyses have been conducted on the macromammal remains found in Sedimentary Unit 7 of the lower Pleistocene Pirro 10 site of the Pirro Nord locality in Italy. Recent field research has concentrated on investigating the fossil content of this site and the genesis of the karst network and its related deposits. In the present study, we distinguish between preburial and synsedimentary (during burial) processes in order to group the vertebrate remains into different taphonomic categories according to their various states of preservation, thus introducing the concept of taphorecord—borrowed from invertebrate taphonomy—into macrovertebrate taphonomy for the first time. Element quantification, evaluation of transport effects, and state of preservation of the skeletal elements were studied and all elements separated into four taphorecords according to their relative chronology. The use of taphorecords also improves the accuracy of the data obtained from the various methods of element quantification. The analysis of bone sorting and orientation indicates that the fossiliferous deposits originated by water flows carrying both fossils and sediments, alternating with phases of carcass accumulation and reelaboration of bones from previously deposited layers. The study presented here serves as a proof of concept for the use of taphorecords in analyses of fossiliferous deposits in caves.

Taphonomic gradients inMiddle Jurassic ammonites of the Iberian Range (Spain)

Abstract Three categories of ammonites fossils have been distinguished in Middle Jurassic of the Iberian Range, according to their state of preservation: reworked or reelaborated elements, resedimented elements and accumulated elements. The relative frequency of reelaborated vs. resedimented ammonites reflects the trends of the hydrodynamic energy in the sedimentary palaeoenvironments, as well as the polarity of the sequences and their discontinuities. Four categories of abrasion surfaces developed on concretionary internal moulds of ammonites are of interest as paleobathymetric indicators: annular furrows, ellipsoidal facets, roll facets, and truncational facets. The transition from associations constituted by accumulated or resedimented ammonites, as well as from associations of resedimented or reelaborated ammonites with truncational or roll facets, to associations of reelaborated ammonites with ellipsoidal abrasion facets or annular abrasion furrows, is interpreted as a taphonomic cline indicative of shallowing gradients from subtidal to inter- or even supratidal environments.