Discovery of Late Jurassic fossils inside modern sediments at Gorringe Bank (Eastern Atlantic Ocean) and some geological implications

Autor: Giovanni Pallini, Maria Alessandra Conti, Giovanni de Alteriis, Renato Tonielli, Maria Marino
Rok vydání: 2004
Předmět:
Zdroj: TERRA nova
16 (2004): 331–337. doi:10.1111/j.1365-3121.2004.00570.x
info:cnr-pdr/source/autori:Conti M. A.; de Alteriis G.; Marino M.; Pallini G.; Tonielli R./titolo:Discovery of Late Jurassic fossils inside modern sediments at Gorringe Bank (Eastern Atlantic Ocean) and some geological implications/doi:10.1111%2Fj.1365-3121.2004.00570.x/rivista:TERRA nova (Print)/anno:2004/pagina_da:331/pagina_a:337/intervallo_pagine:331–337/volume:16
ISSN: 1365-3121
0954-4879
Popis: IntroductionIn the Horseshoe chain, mainly com-prising volcanic seamounts, the Gor-ringe Bank (36 N, 11 W; hereafterreferred to as GB) is an exception,because it consists of suboceanic man-tle peridotites and gabbros. The otherpeculiarity of this seamount is thegreat bathymetric gradient betweenthe summit of the bank, at )30 m,and the surrounding abyssal plains,Tagus and Horseshoe at )5000 m(Fig. 1). The GB is a N60 -trendingridge, 200 km long and 80 km wide,with two summits, the Gettysburg inthe south-west and the Ormonde inthe north-east (Beauzart et al., 1979).The two summits, separated by a deep(800 m) saddle, consist of igneousrock outcrops with very thin or absentsedimentary cover.The first exploration of the GB wasmade by the Glomar Challenger in1970 (120 DSDP sites; Ryan et al.,1973). This and some subsequentexplorations with manned submersi-bles along the flanks of the ridge in the0–3000 m bathymetric range (Auzen-de et al., 1978; Lagabrielle andAuzende, 1982), and more recently atgreater depths (Girardeau et al.,1998), have indicated that the GBrepresents a remarkable example of aseamount comprising lower oceaniccrust⁄upper mantle rocks.In December 2002 to January 2003duringtheGorringe_2003 expedition,on board R⁄V Urania (NationalResearch Council, Italy), swath bathy-metry, seismo-magnetic survey, exten-sive bottom samplings and some ROV(remotely operating vehicle) dives overthe top of the GB were carried out (deAlteriis et al., 2004). A further aim ofthe cruise was to study the bioclasticsediments recorded during previousexpeditions (Auzende et al., 1978; Gir-ardeau et al., 1998) at shallow depths.The location and physiography of theGB guaranteed that these sedimentswere totally autochthonous anduncontaminated by other sources.The only previous analyses availablefromthesesedimentswereprovidedbyAvila and Malaquias (2003).Geological and stratigraphic resultsThe multibeam and geophysical sur-vey, extended to 500 m depth overboth summits, revealed that Gettys-burg seamount has an almost subcir-cular summit, elongated N–S for8–10 km. The shelf area is somewhatconvex from )60 to )175 m. At thecentre of the summit the igneousbasement crops out between depthsof )29 and )60 m. The rocky outcropconsists of roughly N–S-trending rid-ges consisting of foliated serpentinitesand gabbros completely surroundedby a blanket of bioclastic sands andgravels down to )140 m, where a cleardepositional shelf break appears(Figs 1 and 2). Pockets of these sandsare also hosted in the inter ridgedepressions. Sands consist of skeletalcalcareous algae, forams, bryozoansand molluscs. The heterozoan associ-ation (James, 1997) is dominated byForamol facies (Lees and Buller,1972) over the entire shelf area withthe typical absence of allochthonousnon-skeletal grains. Some exhumedEarly Pliocene foraminifers, Globoro-talia margaritae (Bolli & Bermudez),are also present in the fine fraction.Lithoclasts are generally rare andmainly consist of serpentinized ultra-mafics and gabbros.Samplings and ROV observationsconfirm that modern bioclastic mate-rial is continually produced by thebenthic ecosystem that characterizesthis and other Atlantic seamountsas temperate carbonate factories.Detailed bathymetry and subbottomprofiling allow us to establish that theaverage thickness of this material doesnot exceed a few metres in the shelfareas and in the pockets. TheseABSTRACT
Databáze: OpenAIRE
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