Upper Barremian–lower Upper Aptian hemipelagic deposits of the Hamada Formation in the Djebel Serdj area, north-central Tunisia were studied in detail with regard to microfacies, biostratigraphy, δ13C stratigraphy, and geochemistry. Our data provide insights into the palaeoenvironmental evolution and sea-level fluctuations of the Tunisian shelf. The unusually expanded deposits consist of mud-, wacke-, and packstones which reflect mid- and outer-ramp depositional environments. Planktonic foraminifer and δ13C stratigraphy allowed us to establish a detailed time-frame and the recognition of the Lower Aptian Oceanic Anoxic Event 1a (OAE 1a) as well as time-equivalent deposits of shallow-marine carbonate-platform drowning. Based on our microfacies studies, we subdivide the studied sections into four genetic intervals: a pre-OAE 1a interval, an OAE 1a and platform-drowning-equivalent interval, and a post-platform-drowning interval. We present a 3rd-order sea-level curve for the Tunisian shelf, deriving from the results of our microfacies studies.
Deposits of the OAE 1a in the sections investigated are characterised by bioclastic wacke- and packstones with high abundances of poorly preserved radiolarians and moderately to well preserved planktonic foraminifers, suggesting a transgression and an eutrophication of the upper water column. Scarceness of benthic macrofossils, low abundances of small benthic foraminifers and ostracods possibly suggest dysoxic conditions at the seafloor. Mudstones of the platform-drowning time-equivalent deposits, directly overlying the OAE 1a, are partly showing a pronounced drop in carbonate content and are scarce of macrofossils. 相似文献
Two new species and a new genus of scorpionflies of the family Kaltanidae, Pseudochorista occidentalis sp. nov. and Kamochorista novokshonovi gen. et sp. nov. (Mecoptera: Kaltanidae), are described from the Urzhumian of Udmurtia (Chepanikha locality). This is the first record of kaltanids on the Russian Platform. 相似文献
Summary The Triassic platforms of the Dolomite Alps of northern Italy are famous for their well-preserved platform flanks. We report
on the discovery of extensive automicrite on these platform slopes. Automicrite stands for autochthonous micritic carbonate,
formed by in-situ precipitation that was mediated by organisms. The automicrite occurs on the 400 m high, 25°–35° dipping
slopes of the Sella platform that were pervasively dolomitized. Textures include peloidal aggregates (thrombolitic microfabric),
irregular spar-filled cavities as well as biodetritus and are in many samples clearly visible despite the dolomitization.
The peloids may occur as loose particles floating in lighter-colored cement or form a loosely connected framework. The preservation
of automicrite textures along with the nearly unaltered slope geometry of the platform lead to several conclusions on the
development of automicrite on such steep and high platform slopes.
1)
Automicrite formed patches and layers from the platform top down to over 200 m on the flank and constitutes about 25% of the
volume of slope sediment.
2)
Automicrite alternates with layers of skeletal and lithoclastic rubble and sand.
3)
Despite the extensive occurrence of automicrite, no mud mounds developed on the steep slopes. The autochthonous carbonate
only forms layers and cushions that do not significantly alter the planar clinoforms.
4)
On the slope, layers of automicrite slid, became fragmented and turned into breccias that dominate the lower slope. The undolomitized,
distal ends of these breccias contain the Cipit boulders extensively described in the past.
Summary Reef facies, reef types and their biotic associations in the Maiella platform margin (central Italy) provide qualitative evidence
for a significant reef decline across the Cretaceous/Tertiary (K/T) boundary, and indicate two phases of reef recovery during
the Paleocene. Rudists dominated the reef community until the latest Cretaceous. A significant sea-level fall around the time
of the K/T boundary is documented by a truncation surface associated with emersion.
During sea-level highstands in the Danian to Early Thanetian and, more extensively, during the Late Thanetian, coral-algal
patch-reefs grew along the platform margin and top. Already in the Danian to Early Thanetian, the reef communities were more
diverse and the constructional types more evolved than previously known from this time. Differences between the Danian to
Early Thanetian coral association, the Late Thanetian association, and Late Cretaceous coral faunas may have ecological or
evolutionary causes.
Repeated emergence produced a complex diagenetic history in the Danian to Lower Thanetian limestones. All Paleocene reefs
were displaced by gravitative redeposition. Coral-algal reefs are less important in the Early to mid Eocene, when alveolinid
foraminifera dominated on the Maiella shelf. Reefs on the Maiella platform diversified and attained large sizes in the Late
Eocene to Early Oligocene, as known from other Mediterranean platforms.
The external controls on the Late Cretaceous to Oligocene evolution and demise of reef communities that are most easily demonstrated
with our data are sealevel fluctuations and climate change. We propose that the change in reef biota and reef types across
the K/T boundary and during the Early Tertiary were important causes of the parallel changes in platform growth style. 相似文献
Summary The Upper Triassic carbonates of the area comprised between Maratea (Lucania) and Praia a Mare (Calabria) have been studied.
They have been grouped into six facies assemblages which, in turn, define two depositional systems.
1)
a platform margin depositional system, comprising algae-bivalves-bearing dolomites (A1), Megalodontid-bearing limestones and
dolomites (A2), dololutites and stromatolitic/bioclastic dolomites. (A3);
2)
a slope to basin depositional system comprising: buildup-facies, talus breccia and coarse to fine doloarenites (B1), coarse
to fine doloarenites and dark laminated dololutites (B2), dark laminated dololutites (B3).
The algal and bivalve dolomites represent typical peritidal platform sediments arranged in cyclical patterns formed as shoal
barrier and back-reef deposits periodically emerged and deformed in tepee structures.
The outer margin of the platform was colonised by peculiar bioconstructions dominated by microbialites and serpulids; little
sponges occur less frequently, whereas rare corals have been found only in some turbiditic beds.
Along the slope, in a distance that can be estimated in 5 or 6 km, the coarse breccias give way to coarse turbidites, then
finer turbidites and finally to dark, organic-rich micrites. Most of the floatstone and associated turbidites imply a fault-
controlled slope. The laminated texture and the high O.M. (Organic Matter) content of the latter facies quite obviously point
to a basin with poorly oxygenated bottom waters.
In the uppermost Triassic there is an indication of a regressive trend, evidenced by the progradation of the platform facies.
At that time a back-reef area was characterised by Megalodontid prairies alternated to areas of sandy bioclastic transport.
However, due to the poor biostratigraphic record and intense cataclasis, no definite evidence of a well-structured Rhaetian
platform margin with buildups and related basinal sediments exists.
These sedimentary and paleoecological features match well with those found in the intraplatform basins of the Dolomia Principale,
as well as with other coeval facies in Southern Apennines, Central Apennines, Southern Alps and in Southern Spain. Altogether,
this evidence suggests that in the Norian time both Southern Alps and Apennines experienced a very similar tectonic evolution
and comparable paleoceanographic conditions, characterised by the formation of numerous intraplatform throughs with restricted
circulation, the latter influencing the bottom conditions as well as the marginal communities. The margins of these restricted
basins, differently from the typical Dachstein reefs of the Upper Triassic Tethys, were dominated by microbial-serpulid communities. 相似文献
The base of the Rhaetian stage (Norian/Rhaetian boundary, NRB) is still awaiting formal designation by the International Commission on Stratigraphy. At present, only the 4.30‐m‐thick Steinbergkogel section (Austria) has been proposed as GSSP (Global Stratotype Section and Point) candidate for the base of the Rhaetian. Here we present data from the 63‐m‐thick Pignola‐Abriola section (Southern Apennines, Italy) that we consider an alternative candidate for the Rhaetian GSSP. The Pignola‐Abriola basinal section, represented by hemipelagic–pelagic carbonate successions belonging to the Lagonegro Basin, matches all the requirements for a GSSP: 1, it is well exposed with minimal structural deformation; 2, it is rich in age diagnostic fossils (e.g. conodonts and radiolarians); 3, it yields a geochemical record suitable for correlation (e.g. δ13Corg/carb); and 4, it has a robust magnetostratigraphy and is correlated with the Newark APTS for age approximation of the NRB and additional Rhaetian bioevents. In the Pignola‐Abriola section, we opt to place the NRB at the 44.4 metre level, coincident with a prominent negative shift of ca. 6‰ of the δ13Corg. This level is located 50 cm below the FAD of conodont Misikella posthernsteini s.s within the radiolarian Proparvicingula moniliformis Zone. Both the negative δ13Corg shift and the FAD of Misikella posthernsteini occur within Pignola‐Abriola magnetozone MPA‐5r, at ~205.7 Ma, according to magnetostratigraphical correlation to the Newark APTS. We also illustrate the coeval Mt. Volturino stratigraphical section deposited below the calcite compensation depth (CCD) within the same Lagonegro Basin and characterized by a detailed radiolarian biostratigraphy and strong δ13Corg negative shift around the NRB. 相似文献
The Middle Albian sequence from the western marginal area of the Vasco-Cantabrian Basin contains calcified microbialites in
different marine depositional environments, individually well defined by microstructure, lamina characteristics and mode of
formation. Microbialites may form the primary framework of reefs, which occur as composite stacks in mid to lower slope environments
or as isolated bodies in small intraplatform basins. In most areas microbialite reef growth was initiated below the photic
zone. Stratiform intercalations of microbialites and composite microbialite/foraminifer oncoids are restricted to well bedded
carbonate platform deposits (Urgonian). Three basis types of microbialites are recognized:
(i)
Dense micritic/fenestral microbialites corresponding to laterally linked, stacked stromatolitic hemispheroids. The development
and preservation of stromatolitic structure is a function of sediment supply and secondary obliteration by succesive boring
activities. They were calcified in situ at the surface with irregularly curved linings of microcrystalline carbonate. Dense
micritic/ fenestral microbialites, variously developed and preserved, are the main contributors to microbialite reefs. Microbialites
form hard substrates bored by lithophagous pelecypods and boring sponges (Aka sp.). The main associated faunal elements include lithistid and coralline demosponges, hexactinellid sponges, encrusting
foraminifera, brachiopods, polychaetes, and bryozoans.
(ii)
Dense micritic/peloidal microbialites with subplanar, arhythmic lamination (binding habit). They were calcified in situ below
the surface in conjunction with decaying organic matter. At large scale, they occur in shallow water, i.e. within the photic
zone. They cover earlier microbialite reefs or occur on and in episodic deposits of coarse biodebris. At small scale they
occur in protected microenvironments (e.g. intraparticle space, boring cavities).
(iii)
Peloidal/in situ ooid microbialites with subplanar/ wavy lamination occur as small-scale stratiform intercalations in carbonate
platform deposits, episodically revealing physical reworking. Other features are very similar to dense micritic/peloidal microbialites.
The results of geochemical analyses indicate a rock-buffered diagenetic system during early diagenetic and burial history
of microbialite reefs. Independent of microbialite type residual MgCO3-contents are in the range of 1.20 to 3.57 mole %, agreeing well with those from isopachous rim cements and indicating a high
Mg-calcite precursor of microbialite micrites. Stable isotope values (δ13C) are in the range of 3.13 to 3.80 (permil, vs PDB), close to the internal standard, the coralline spongeAcanthochaetetes (Albian species=2.93; Recent species=3.27) and comparable with inorganically precipitated Mg-calcite. 相似文献
We study conodonts from the upper part of the Shizhou Member of the Yujiang Formation in Liujing area for the first time. Relatively abundant conodonts were collected from six sampling levels located between the uppermost part of the Shizhou Member to the lowermost part of the Daliancun Member at the Shizhou section, Liujing, Guangxi. The material includes Polygnathus excavatus excavatus Carls and Gandl, P. excavatus ssp. 114 Carls and Valenzuela-Ríos, P. nothoperbonus Mawson, P. perbonus (Philip) as well as ?Ozarkodina? midundenta (Wang and Ziegler) and ?O.? prolata Mawson. The lowest record of P. nothoperbonus is documented 0.35 m below the boundary between the Shizhou and Daliancun members. Correlation of conodont biostratigraphy between the Shizhou and Liujing sections indicates that the upper part of the Shizhou Member (partially) belongs to the excavatus Zone and the Daliancun Member belongs to the nothoperbonus Zone. 相似文献