documenta naturae abhandlungen    ISSN (Print) 2626-4161 - ISSN (Online) 2626-9864

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Band 1 (2018) Teil 1 (Textband) & 2 (Tafelband)

 

Mikropaläontologie (Foraminiferen, Ostrakoden), Biostratigraphie und fazielle Entwicklung der Kreide von Nordsomalia

mit einem Beitrag zur geodynamischen Entwicklung des östlichen Gondwana im Mesozoikum und frühen Känozoikum

 

Micropalaeontology (Foraminiferida, Ostracoda), biostratigraphy and facies development of the Cretaceous of Northern Somalia

including a contribution concerning the geodynamic development of eastern Gondwana during the Cretaceous to basal Paleocene

 

Peter LUGER (†)

 

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Kurzfassung

Erstmals werden die kretazischen bis altpaleozänen Mikrofaunen (Foraminiferen, Ostrakoden) des zentralen Nordsomalias (zentrale und östliche Ahl Medo- und westliche Ahl Meskat-Küstenkordilliere) vollständig monographisch erfasst. Diese entstammen einer dichten Beprobung von insgesamt acht übertage aufgeschlossenen Profilen, wobei die Analyse der Mikrofaunen stratigraphisch den Zeitraum vom basalen unteren Apt bis zum mittleren Cenoman, und nach einem deutlichen Hiatus Campan, Maastricht und unteres Paleozän ergibt. Insgesamt werden 84 Foraminiferengattungen mit insgesamt 119 Arten (inkl. 6 neu errichteter Arten/Unterarten) und 65 Ostrakodengattungen (inkl. Untergattungen) mit insgesamt 152 Arten (inkl. Unterarten) beschrieben und dokumentiert. Bei den Ostrakoden werden 4 neue Gattungen und 24 neue Arten (inkl. Unterarten) eingeführt. Die biostratigraphische Auswertung ergibt für das Apt acht, für das Alb vier, das Cenoman drei, das Campan und Maastricht jeweils eine und das untere Paleozän drei aufeinander folgende typische Assoziationen von Foraminiferen und Ostrakoden (auf die formale Errichtung von Biozonen wird wegen der starken faziellen Abhängigkeiten der jeweiligen Leitfossilien verzichtet). Ökologisch sind die basalen Sedimente der Kreide dem Apt in limnischer Fazies zuzuordnen, die bereits im tiefen Unterapt in Ablagerungen einer Karbonatrampe übergehen. Erstmalig werden für das untere Oberapt Ablagerungen einer offenen Schelffazies aus Nordsomalia beschrieben (mit planktonischen Foraminiferen: Leupoldina cabri-Zone und reichhaltiger Ammonitenfauna), die bereits im höheren Oberapt wieder in die zunächst bis zum mittleren Cenoman anhaltende nach NW verlandende Karbonatrampenfazies übergehen. Auch die karbonatischen Sedimente des Campans, Maastrichts bis unteren Paleozäns sind in dieser Fazies entwickelt. Der allgemeine geologische Rahmen Somalias mit seinen tektonischen Elementen und der sedimentären Entwicklung bis zum Paläogen wird kurz dargestellt. Besonders ausführlich wird die Ablagerungsgeschichte Nordsomalias in der Kreide beschrieben und in den acht Profilen graphisch abgebildet. Anhand der Faunenanalyse wird die Ökologie der kretazischen Sedimente diskutiert sowie die Paläobio-geographie der Faunenelemente dargestellt. Letztere ergibt für das Apt bis Cenoman eine zunehmende Vereinheitlichung sowohl bei den Foraminiferen als auch bei den Ostrakoden im südtethyalen Raum, wohingegen im Campan bis unteren Paleozän bei den Foraminiferen eine zunehmende Trennung in west- und osttethyale Faunenelemente zu beobachten ist. Noch deutlicher ist bei den Ostrakoden ab dem Coniac bis zum unteren Paleozän eine strikte Trennung zwischen einer nordafrikanisch/levantinischen und einer iranisch/arabisch/somali-schen Faunenprovinz erkennbar. Ein Vergleich der faziellen Entwicklung der Kreide von Somalia und der nördlich und südlich angrenzenden Gebiete und deren geodynamischer Geschichte wird angestellt. Hierbei zeigt sich deutlich, dass Nordsomalia während der Kreide sowohl in der faziellen Ausbildung als auch in der geodynamischen (sequenzstatigraphischen-tektonischen) Entwicklung hohe Übereinstimmungen mit der arabischen Plattform aufweist. Hierbei sind jedoch die Hiaten zwischen Jura und Kreide sowie zwischen Cenoman und Campan deutlich länger als in den zu dieser Zeit direkt nördlich angrenzenden Gebieten, weshalb Nordsomalia als Südrand der arabischen Platte interpretiert wird. Der lange Hiatus zwischen dem obersten Jura und dem Apt in Nordsomalia in Kombination mit starker vertikaler Tektonik und lokal völliger Erosion jurassischer Ablagerungen wird mit dem Ende der Süd-Drift Madagaskars und dem Einsetzen der Drift zwischen Afrika/Madagaskar und Antarctica/ Australien in der tieferen Unterkreide in Verbindung gebracht. Der deutliche Hiatus zwischen dem Cenoman und Campan mit lokaler Erosion cenomaner Sedimente steht in klarer Verbindung mit dem Rift und der einsetzenden Drift zwischen Madagaskar und der Indo/Seychellen-Platte im Turon.

 

Schlüsselwörter

Nordsomalia, Kreide, Mikropaläontologie, Foraminifa, Ostrakoda, Geologie, Geodynamische Faktoren, Zerfall von Ostgondwana.

 

 

Extended summary

The present study represents the first attempt of a monographic micropalaeontological study on the Cretaceous to Early Paleocene Foraminifera and Ostracoda of central northern Somalia. The investigated samples derived from eight surface sections taken by the author in the central/eastern Ahl Medo and western Ahl Meskat Mountains, covering about 2050 meters in total. These studies had been carried out in the late 1980ies in the scope of the “Special Research Project 69” hosted at the Technical University of Berlin and funded by the German Research Foundation. According to the micropalaeontological results, the Cretaceous so called “Tisje-Formation” (name disputed) could be subdivided into four units of Aptian to Cenomanian age and a fifth unit of Campanian age, unconformably overlying differently deep eroded Cenomanian sediments. Subsequent to a short regressive phase, this is followed by the conformably overlying Maastrichtian to Early Paleocene calcareous deposits of the lower Auradu Formation (of which the thick overlying Paleocene and younger parts are not included here).
Within the entire collection of the investigated Cretaceous to Early Paleocene microfossils, 84 genera with altogether 119 species (including 6 new species) of Foraminifera and 65 genera with overall 152 species of Ostracoda (of which 4 genera and 24 species are newly introduced) could be recognised. Species with only rare or poorly preserved specimens are often left in open nomenclature. Most of the listed specimens are described or discussed, all are figured, either as REM-photos (Foraminifera and Ostracoda) or light micros-copic photographs from thin-sections (Foraminifera) on 49 plates at the end of the text part of this study. The material is kept in within the collection of the SFB 69 (which will be transferred to the “Museum für Naturkunde” in Berlin in the next future).

The base of the Cretaceous sedimentary sequence is made up of an up to ~10 m thick conglomerate (see fig. [Abb.] 15) carrying pebbles and cobbles of reworded parauthochtonous limestones among other material and unconformably overlying either deeply weathered (and locally karstified) Late Jurassic deposits or pan-African basement, which was uplifted and eroded during the long Hiatus between the Tithonian and Aptian. However, calpionellids found within some of the pebbles or cobbles within these conglomerates are of Late Tithonian to Early Valanginian age (LUGER et al., 1996) and may therefore indicate primarily continuous marine sedimentation until the very Early Cretaceous. In this case, the unconformity would have to be placed within the Early Cretaceous and probably correspond to the “late Valanginian unconformity” observed and pointed out by SHARLAND et al. (2004) for the Arabian Peninsula (see below and fig. 43 of the present study). The overlying sequence of calcareous (east Ahl Medo) to silicilicastic (central Ahl Medo) dominated sediments are of limnic origin and carry a fauna of freshwater ostracods and fauna of charophytes (unit 1 of Tisje Formation), of which the latter indicate a basal Aptian age (SCHUDACK in : LUGER and SCHUDACK, 2001). As an inconvenient result, some of the marine microfossils within the conformably overlying shallow marine strata, formerly believed to indicate a Late Barremian age now have to be placed within the very Early Aptian. The latter study is the first biostratigraphical investigation on this so-called “Somali Wealden”. Some of the more prominent horizons indicating subaerial exposure within the calcareous eastern Ahl Medo (e.g. Xabaal Garure section, see fig. 11) may be misinterpreted to represent major unconformities. However, their frequent occurrence within this very shallow facies is actually to be expected and therefore most probably only indicates very short-lived events.

Upwards the sequence grades into shallow marine, mostly calcareous deposits of Early Aptian age (unit 2 of Tisje Formation; biostratigraphic associations A2 to A6, see: chapter 6 and table 7) carrying increasingly diversified microfaunas of (larger) Foraminifera accompanied by calcareous algae; macrofaunal elements often are corals, rudists and other pelecypods and gastropods. A very conspicuous horizon is formed by the mass-occurrence of large Valserina transiens CHERCHI und SCHROEDER (originally believed to be of Late Barremian age), which dominates association A5 of the present study. It is assumed here, that the latter species invaded from other areas into northern Somalia, as it occurs for the first time already with large specimens. Also, its assumed predecessor Valserina charollaisi (see: CHERCHI and SCHROEDER, 1999) is absent in the underlying marine sediments. Altogether these deposits are interpreted to represent a north-westerly shoaling carbonate ramp facies during the early Aptian (the thick lower unit 2 of Tisje Formation). It is agreed to assume that Valserina transiens CHERCHI und SCHROEDER forms the predecessor of Palor-bitolina lenticularis BLUMENBACH, with which it co-occurs in its uppermost part of its episode and is later on replaced by it (here: interval A6). Since no isolated microfossils could be extracted from the intervals A2 to A6, no ostracods can be reported from this sequence.

Another very important event, reported for the first time from northern Somalia within the scope of this study, is indicated by the overlying marls and limestones with large ammonites and a rich hemipelagic microfauna including moderately diversified planktonic and highly diversified smaller benthonic Foraminifera as well as Ostracoda (for detailed composition see chapter 6: interval A7). These only rarely well exposed deposits (best example: Xabaal Garure section, fig. 11, chapter 4.2) are rapidly tapering out westwards, and are assumed to be equivalent to the shallow marine deposits with Palorbitolina lenticularis BLUMENBACH, which are directly overlying the pan-African basement at Erigavo (“Erigavo-Mukalla high”, see chapter 3). Due to the composition of the planktonic Foraminifera this entire interval may be attributed to the Leupoldina cabri zone of early Late Aptian age. Upwards a shoaling and a return to the carbonate ramp facies is observed. Among the various subfacies within the upper unit 2 of Tisje Formation - dominated corals and rudists among many other macrofossils - it is again the Xabaal Garure section, in which a reefal complex of dendroid hermatypic corals in nicely exposed in situ (see fig. 17, chapter 4).

A sharp lithological contact is observed in between the Aptian and the overlying (probably not basal) Early Albian marls and limestones of the basal unit 3 of Tisje Formation (biostratigraphic association B1), assumed to represent a minor hiatus observed also in the Arabian Peninsula (SHARLAND et al., 2004; see below and fig. 43 in chapter 9 of the present study). In most of the studied sections this interval, although carrying rich marine micro- and macrofossil faunas, surprisingly does not contain Foraminifera but ostracods. Only in two section the name-giving larger foraminifer Mesorbitolina texana (ROEMER) sensu SCHRÖDER (1985) directly occurs already in the basal sediments of unit 3 (see chapter 6.2), whereas it appears later in the other sections covering this interval. Together with the overlying mostly fossil-rich mainly calcareous sediments of shallow marine provenance again the facies of a north-easterly shoaling carbonate ramp was established, (see fig. 36 in chapter 9.1). The northeast-ward shoaling of this carbonate ramp and its interfingering with the Yesomma sandstones of north-western Somalia is demonstrated in fig. 38 in chapter 9.1. This situation lasted a least until mid-Cenomanian times (unit 4 of Tisje-Formation, biostratigraphic associations B2 to B4, C1 to C3, for detailed composition of each assemblage see chapter 6), after which widespread uplift occurred in northern Somalia. This uplift caused widespread erosion of Cenomanian deposits in most of the investigated sections (the most complete record is again observed only in the Xabaal Garure section see fig. 11, chapter 4.2).

Following the Turonian to Santonian uplift, the investigated area subsided again in Campanian times - here treated as unit 5 of Tisje Formation. This unit can be subdivided into three clearly defined sedimentary sequences in the eastern-more Ahl Medo and Ahl Meskat (see e.g. figs. 19, 22 in chapter 4). Subunit 5a is made up of fluvial to shallow marine siliciclastics in a continuous sequence grading upwards in the predominantly calcareous, highly fossiliferous sediments of unit 5b (thinning out towards the north-west). It is especially rich in highly visible large bioherms of rudists (see fig. 21) among corals and other macrofossils and an even a more diversified microfauna of larger Foraminifera (among others Orbitoides tissoti SCHLUMBERGER sensu NEUMANN, 1987) and enormously rich assemblages of Ostracoda (altogether at least 34 species), which clearly indicate a Campanian age (for detailed composition of this assemblage see chapter 6: Interval D1). Upwards these deposits are followed by a regressive/transgressive cycle of siliciclastis (unit 5c of Tisje Formation) which grade - without visible sedimentary break - again into the highly fossiliferous calcareous lower Auradu Formation, likewise representing the facies of a carbonate ramp. If it is not the highly visible macrofauna of various rudists among corals, other pelecypods and else alone, it is at last the as well highly visible fauna of larger Foraminifera (with large specimens of Loftusia spp.), which clearly assigns a Maastrichtian age to these deposits, also rich in Ostracoda (for detailed composition of this assemblage see chapter 6: Interval E1) at least in the eastern-more central Ahl Medo and Ahl Meskat. As reported earlier by LUGER et al. (1990, 1994) the KT-boundary is only well exposed in one of the investigated sections (at Tisje, western Ahl Meskat, see fig 20), where it occurs in the lower part of thick massive limestones and is only marked by an abrupt termination of the Maastrichtian faunal elements, which are replaced by only poor faunas of smaller foraminifera and calcareous algae (macrofossils are hardly to be observed; see interval F1, chapter 6:). CHERCHI et al. (1993) described a similar situation for an eastern-more section in the Ahl Meskat Mountains. It is assumed though highly unanticipated in this shallow marine facies that in these sections the sedimentation was continuous during the K/T-boundary, since within an overlying marly clay horizon, a diverse microfauna of middle Early Paleocene (Danian) age could be recovered (for the first time in northern Somalia, see interval F2 in chapter 6). Upwards follow thick-bedded, massive limestones with at first moderately diverse microfaunas (see interval F3 in chapter 6) and relatively poor macrofossils. As already mentioned, the thick higher part of the Auradu Formation is not included in this study.

The total sedimentary sequence of central northern Somalia following the biostratigraphic investigations of METTE (1993: Jurassic) and the present study are displayed in part B of fig. 38 (chapter 9.1, for a coloured version see LUGER and DOMINIK, 2009), including the north-western facies development as given by BRU-NI and FAZZUOLI (1980) and BOSELLINI (1989) by surface and well data (part C of fig. 38). There is a conspicuous discrepancy to be observed within the Cretaceous subdivision and stratigraphic attributions within the well data presented by BOSELLINI (1989, part A in fig. 38), little to the south of the studied area. As already pointed out by LUGER and DOMINIK (op. cit.), there is hardly any correlation with the sections given by BOSELLINI (op. cit.), who stated only lithostratigraphic units and ages - without giving any palaeontological data. It is in particular the absence of any Aptian sediments, the stated presence of a “pelagic Turonian” (while nowhere in Somalia marine Turonian deposits had been proved until now), thick “lower Senonian” sediments and the absence of marine Campanian to (Early) Paleocene in northern Somalia which is in obvious and unexplainable contradiction to the paleaeontological data obtained from the surface section presented here (Part B of fig. 38).

The implementation of the Cretaceous to Early Paleocene data from northern Somalia into the chronosequence-stratigraphic chart as given for the Arabian Peninsula by SHARLAND et al. (2004) and southern Iran provided by JAMES and WYND, 1965) as shown in fig. 43 (chapter 9.2.3; for a coloured version see LUGER and DOMINIK, 2009) displays the high similarities of the sequential arrangement and facies development of the (during the Cretaceous) still connected directly northerly bounded areas. The longer lasting hiata during the Early and Late Cretaceous become explainable, if a southerly “rimming” position of northern Somalia during these times was accepted (see LUGER and DOMINIK, 2009 for a more detailed discussion).

The “late Valanginian unconformity” on the Arabian Peninsula - much more pronounced in northern Somalia - is synchronous to the end of the southwards drift of Madagascar as given by COFFIN und RABINOWITZ (1992) as about 130 Ma (~Valanginian). In this context it is important that POWELL et al. (1988) and LAWVER et al. (1992) place the break-up and beginning drift between the Antarctic and Australian plates and the Malgache/Indian plate during the latest Jurassic and very Early Cretaceous. This event might have had severe consequences onto the northerly neighbouhring plates in uplift and subsequent subsidence.

The reason for the widespread hiatus between the Late Aptian/Early Albian in Somalia and the Arabian Peninsula is not yet understood.

The opposite is true for the hiatus widely observed during the middle Turonian on the Arabian Peninsula, southern Iran and particularly in northern Somalia, where it has a much longer extent (as it is the case in some other parts of the regions mentioned before). This clearly correlates with the break-up and initial drifting between the Madagascar and the Indo/Seychelle plates. It is widely accepted that this event occurred during the “middle” Turonian, accompanied by Early and Late Turonian basaltic lava flows on the Madagascar plate. However, the pathway of the Indo/Seychelle plate is still under discussion. While SCOTESE, 1991, 2001a, b; SCOTESE und GOLONKA, 1992, SCOTESE et al., 1988 u.a.) until today suggest an isolated island situation for it in their plate-tectonic reconstructions (e.g. http://www.scotese.com/1202d.htm [28/05/08]). BOSELLINI (1989, 1992) and especially CHATTERJEE (1992) suggest this “pathway” having been much closer situated in neighbourhood to the Afro/Arabian plate. It was BOSELLINI (1989), who first demonstrated the four-fold subdivision of the Somali coastal margins (reproduced here as figs. 46, 47 in chapter 9.4) and pointed out the peculiarities of the “Late Cretaceous Paleogene left lateral sheared margin” north of the El-Hamurre Lineament. He pointed out the importance of the Owen-fracture-zone, along which the Indo/Seychelle plate should have drifted northwards. CHATTERJEE (1992) used terrestrial vertebrate remains from the infra- and intra-trappean beds (Maastrichtian) of India to emphasise their close relations to Eurasian forms. He suggested a terrestrial connection with Eurasia via Sokotra (now island) in Late Cretaceous times, and Sokotra as a pivot point around which the Indo/Seychelle plate should have rotated counter-clockwise before drifting into its present position.

The importance of the separation event between Madagascar and India is also observed herein by the dramatic change of the shallow marine benthonic microfossil assemblages. This event - the change of the northwest African through Arabian/North Somalian more or less uniform ostracod assemblages during the Cenomanian to strictly separated north African/Levantinian and Irano/Arabian/North Somalian ostracod assemblages since the Coniacian - has already been discussed in detail by LUGER (2003). A similar split, though not that dramatically, in western and eastern Tethyan faunas of larger Foraminfera is observed at the same time. Due to the absence of any data on post-Turonian Ostracoda north of Lebanon in the eastern Mediterranean, it can hence not be seriously discussed, whether this split is due to facial/ecological or other reasons (e.g. change in major oceanographic currents).

 

Remarks: The author intends to publish an English version of the important genera and species of the Foraminiferida and Ostracoda discussed herein well as an English version of the descriptions of the newly introduced taxa of both groups.

 

Keywords

Northern Somalia, Cretaceous, Micropalaeontology, Foraminifera, Ostracoda, Geology, Geodynamics, Disintegraton of Eastern Gondwana.