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Palaeoecology and Taxonomy of the Macrobenthic Fauna from the Upper Cretaceous Ajlun Group, Southern Jordan

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Palaeoecology and Taxonomy of the Macrobenthic Fauna from the Upper Cretaceous Ajlun Group, Southern Jordan Palaeoecology and taxonomy of the macrobenthic fauna from the Upper Cretaceous Ajlun Group, southern Jordan Abstract. The Upper Cretaceous Ajlun Group (Cenomanian-Turonian) of southern/south-eastern Jordan has been analysed in 15 detailed sections with thicknesses between 40 m and 200 m. Taxonomic, palaeoecological, taphonomic, and sedimentological aspects were taken into account. During the early Upper Cretaceous the study area was situated at the south-eastern margin of the Tethys Ocean, between the palaeo-shoreline in the south-east and an offshore carbonate platform in the west. Thus, the measured sections include a complete facies succession from terrestrial-dominated environments via marginal marine siliciclastics to an area of carbonate precipitation. So far, very little is known about the fauna and the depositional environment of the group, especially of the transitional marginal marine part. Also, in depth studies of the Cretaceous fauna of southern Jordan are very rare. Therefore, the benthic fauna of the area is described in an extensive taxonomic chapter. It consists of 117 taxa, 77 of which are bivalves, 22 gastropods, 9 echinoids, and 4 corals. The phyla Porifera, Bryozoa, and Brachiopoda are represented by 1 species each. Additionally, at least two species of decapod crustaceans were found. One bivalve species is new: Anthonya jordanica from Cenomanian claystones of the eastern study area. 41 quantitative samples of the benthic invertebrate fauna were grouped into nine associations and three assemblages by means of a Q-mode cluster analysis. These are described as remnants of former communities and their environments are discussed. Salinity and substrate consistency are assumed to have been the most important environmental parameters controlling the faunal distribution. The overall palaeo-environment is discussed on the basis of sedimentological and palaeoecological results. It was primarily influenced by the morphology of the sea floor, sediment supply, and salinity of the sea water. ■ Cretaceous, palaeoecology, cluster analysis, benthos, taxonomy, Jordan Zusammenfassung: Mit Hilfe von 15 detaillierten Profilen mit Mächtigkeiten zwischen 40 m und 200 m wird die oberkretazische Ajlun-Gruppe (Cenoman-Turon) im Süden/Südosten Jordaniens auf Fazies- und Faunenverteilung untersucht. Dabei wird besonderes Augenmerk auf Taxonomie, Paläoökologie, Taphonomie und Sedimentologie gelegt. Während der frühen Oberkreide lag das Untersuchungsgebiet am Südostrand der Tethys, im Bereich zwischen Küstenlinie und einer vorgelagerten Karbonatplattform. Die Profile liefern einen kompletten Schnitt vom terrestrischen Ablagerungsraum im Südosten über stark siliziklastisch geprägte randlich marine Verhältnisse zu vollmarinen Bereichen mit Karbonatsedimentation. Insgesamt kann gerade der klastisch dominierte Übergangsbereich als bislang nahezu unbearbeitet gelten. Auch paläontologische Untersuchungen fehlen für dieses Zeitintervall bis auf wenige Ausnahmen nahezu völlig. Daher wurde zunächst in einem taxonomischen Teil eine Bestandsaufnahme der Fauna (mit Schwerpunkt auf Benthosorganismen) gemacht. Für das Arbeitsgebiet konnten 2 Ammonoideen und 117 Benthosarten bestimmt werden, die sich aus 77 Muschel-, 22 Schnecken-, 9 Seeigel- und 4 Korallentaxa, sowie je einen Vertreter der Schwämme, Bryozoen und Brachiopoden zusammensetzen. Zusätzlich wurden mindestens zwei neue Arten decapoder Krebse gefunden. Bei den Bivalven konnte die neue Art Anthonya jordanica sp. nov. aufgestellt werden. Der allgemein mäßige Erhaltungszustand der Fauna ließ die Aufstellung weiterer neuer Arten nicht zu, obwohl unter den in offener Nomenklatur geführten Taxa mit Sicherheit einige neue Arten sind. Nach einer Fazies-Analyse der verschiedenen lithologischen Einheiten wurden mit Hilfe einer paläoökologischen Analyse neun Faunenassoziationen und drei Faunenvergesellschaftungen ausgeschieden. Diese statistischen Einheiten, überwiegend Überreste ehemaliger Lebensgemeinschaften, werden beschrieben und die sie bestimmenden Milieuparameter diskutiert. Die wichtigsten, limitierenden Faktoren, welche die Verteilung der Faunengemeinschaften entscheidend steuern, sind die Salinität und das Substrat. 2 ROMAN BERNDT Eine auf Faziesanalyse und Paläoökologie basierende Rekonstruktion des Ablagerungsraumes zeigt, dass dieser weitgehend von der Topographie des Meeresbodens, vom Sedimenteintrag und der Salinität des Meerwassers beeinflusst war. ■ Kreide, Palökologie, Clusteranalyse, Benthos, Taxonomie, Jordanien Contents 1 Introduction and aim of the study __________________________________________7 2 Previous studies_________________________________________________________7 3 Geological Framework ___________________________________________________8 3.1 Palaeogeographic setting and evolution of Jordan during the Cretaceous ___________ 9 3.2 Stratigraphy_____________________________________________________________ 11 3.2.1 The Kurnub Group ___________________________________________________________ 12 3.2.2 The Ajlun Group _____________________________________________________________ 12 3.2.3 The Belqa Group _____________________________________________________________ 17 4 Facies associations in the Upper Cretaceous of southern Jordan ________________20 4.1 Coastal plain facies association _____________________________________________ 20 4.2 Marginal marine facies associations _________________________________________ 20 Lagoonal facies ___________________________________________________________________ 22 4.3 Transitional facies associations _____________________________________________ 23 Oyster-rich marlstone facies ________________________________________________________ 23 Sandy grain- to rudstone facies______________________________________________________ 25 4.4 Carbonate platform facies association _______________________________________ 25 Foraminiferal, partly laminated mud- to wackestone facies ______________________________ 25 5 Sections ______________________________________________________________30 5.1 Section 1 (At Taiyba) _____________________________________________________ 31 5.2 Section 5 (Ras En Naqb area)_______________________________________________ 33 5.3 Section 9 ________________________________________________________________ 38 5.4 Section 11 _______________________________________________________________ 41 5.6 Biostratigraphic remarks __________________________________________________ 45 6 Sequence stratigraphy and sea level fluctuations _____________________________48 7 Faunal composition and preservation ______________________________________51 7.1 Nekton _________________________________________________________________ 51 7.2 Benthos_________________________________________________________________ 51 7.3 Microfauna______________________________________________________________ 54 7.4 Ichnofauna ______________________________________________________________ 54 7.5 Preservational and taphonomic aspects ______________________________________ 55 7.6 Autecology of the benthic fauna_____________________________________________ 56 8 Palaeoecology _________________________________________________________60 8.1 Introduction and definitions________________________________________________ 60 Palaeoecology and Taxonomy of the macrobenthic from the upper cretaceous Ajlun Group, Southern Jordan 3 8.2 Methods ________________________________________________________________ 61 8.3 Cluster analysis __________________________________________________________ 62 8.4 Diversity ________________________________________________________________ 62 8.5 Benthic associations in the Upper Cretaceous of southern Jordan ________________ 65 8.5.1 Euhaline carbonate platform associations_________________________________________ 65 8.5.2 Euhaline associations of marly limestones/marlstones_______________________________ 70 8.5.3 Euhaline associations of silty claystones and claystones _____________________________ 72 8.5.4 Brachyhaline associations of siltstones/sandy claystones/silty claystones________________ 77 8.7 Comparative analysis of associations and assemblages __________________________ 84 8.7.1 Faunal composition ___________________________________________________________ 84 8.7.2 Life habits___________________________________________________________________ 85 8.7.3 Feeding modes _______________________________________________________________ 86 8.8 Relationship between macrobenthos and environmental parameters ______________ 87 8.8.1 Substrate ___________________________________________________________________ 87 8.8.2 Water energy and bathymetry __________________________________________________ 90 8.8.3 Oxygen _____________________________________________________________________ 90 8.8.4 Salinity _____________________________________________________________________ 91 8.8.5 Climatical aspects ____________________________________________________________ 93 9 Reconstruction of the depositional environment ______________________________94 10 Conclusions __________________________________________________________95 11 Taxonomy ___________________________________________________________96 11.1 Class Bivalvia Linné, 1758 ________________________________________________ 96 Nuculidae GRAY, 1824 Nucula (Nucula) sp. ..................................................................................... 96 Nuculanidae ADAMS & ADAMS, 1858 Nuculana (Nuculana) aff. perdita (Conrad, 1852) ..................................... 96 Mesosaccella cf. subacuta (Stephenson, 1952)
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