Bernd Kaufmann

Mud mounds-a polygenetic spectrum of fine-grained carbonate buildups

SummaryThis research report contains nine case studies (part II to X) dealing with Palaeozoic and Mesozoic mud mounds, microbial reefs, and modern zones of active micrite production, and two parts (I and XI) summarizing the major questions and results. The formation of different types ofin situ formed micrites (automicrites) in close association with siliceous sponges is documented in Devonian, Carboniferous, Triassic, Jurassic and Cretaceous mounds and suggests a common origin with a modern facies found within reef caves. Processes involved in the formation of autochthonous micrites comprise: (i) calcifying mucus enriched in Asp and Glu, this type presumably is linked to the formation of stromatolites, thrombolites and massive fabrics; (ii) protein-rich substances within confined spaces (e.g. microcavities) result in peloidal pockets, peloidal coatings and peloidal stromatolites, and (iii) decay of sponge soft tissues, presumably enriched with symbiotic bacteria, lead to the micropeloidal preservation of parts of former sponge bodies. As a consequence, there is strong evidence that the primary production of micrite in place represents the initial cause for buildup development. The mode of precipitation corresponds to biologically-induced, matrix-mediated mineralization which results in high-Mg-calcites, isotopically balanced with inorganic cements or equilibrium skeletal carbonates, respectively. If distinct automicritic fabrics are absent, the source or origin of micrite remains questionable. However, the co-occurring identifiable components are inadequate, by quantity and physiology, to explain the enhanced accumulation of fine-grained calcium carbonate. The stromatolite reefs from the Permian Zechstein Basin are regarded as reminiscent of ancestral (Precambrian) reef facies, considered the precursor of automicrite/sponge buildups. Automicrite/sponge buildups represent the basic Phanerozoic reef type. Analogous facies are still present within modern cryptic reef habitats, where the biocalcifying carbonate factory is restricted in space.

Temporal–spatial reconstruction of the early Frasnian (Late Devonian) anoxia in NW Africa: new field data from the Ahnet Basin (Algeria)

Anoxic conditions were widespread in NW Africa during the early Frasnian (Late Devonian) that resulted in deposition of organic-rich shales and limestones with total organic carbon (TOC) values of up to 14%. Organic richness and thickness of these sediments vary laterally, and organic-rich vs. organic-poor facies boundaries are likely to have been diachronous. A precise temporal–spatial reconstruction of this anoxic phase in NW Africa is complicated because the organic matter in outcrops is largely oxidised and biostratigraphic resolution in boreholes is generally low due to the lack of recoverable conodonts. This contribution is based on eight outcrop sections at the margin of the central Algerian Ahnet Basin, where detailed spectral gammaray measurements were carried out using a handheld instrument. The pre-weathering organic richness in Frasnian outcrop sections is approximated using the characteristic uranium enrichment in the anoxic facies that, based on well studies, is positively correlated with the total organic carbon content. Conodont biostratigraphic results from these sections suggest that the uraniumenriched interval (the anoxic interval) at the basin margin is most common in the basal Frasnian conodont Biozones 1–2, confirming previous results from the Anti Atlas in Morocco. In three of the eight localities studied the basal Frasnian has not been deposited and the Frasnian here commences with distinctly younger uranium-enriched intervals, including Zones 4–11. Well data from the eastern Algerian Berkine Basin is interpreted to indicate a significantly longer anoxic phase there. Million-yearscale diachroneities of the Frasnian anoxia, therefore, clearly exist across the North Africa shelf. It is assumed that the palaeorelief might have been a major factor in controlling the onset, duration, and intensity of anoxia in the region. D 2003 Elsevier B.V. All rights reserved.

Sedimentary evolution of a Palaeozoic basin and ridge system: the Middle and Upper Devonian of the Ahnet and Mouydir (Algerian Sahara)

The Ahnet and Mouydir regions of southern Algeria are part of one of the worlds largest, almost undeformed exposures of Palaeozoic rocks which exemplify a hitherto poorly known early Variscan development of a Devonian basin and ridge system. This area includes a series of intracratonic basins along the northern margin of the West African Craton which consists (from W to E) of the Reggane Basin, Azel Matti Ridge, Ahnet Basin, Foum Belrem Ridge and Mouydir Basin. The depositional and palaeogeographic interpretation is based on 71 sections in this region, which for the first time were biostratigraphically calibrated by means of conodonts, goniatites and brachiopods. The structural evolution during Devonian times was probably controlled by reactivation of ancient N- S- to NW-SE-running faults in the Precambrian basement, which caused differential subsidence and uplift of a previously largely unstructured siliciclastic shelf. A hiatus during Emsian times indicates widespread emergence during this interval. The entire area was flooded during the earliest Eifelian, when the first vestiges of the Azel Matti Ridge become evident by stratigraphic condensation. The palaeogeographic differentiation is most apparent during the Givetian, when a shoal with reduced carbonate sedimentation was established on the Azel Matti Ridge passing towards the west and east into basinal environments of the Reggane and Ahnet basins, respectively. The Foum Belrem Ridge is distinguished by increased subsidence during the early Givetian and by revived uplift during the late Givetian. In the Mouydir Basin further east, up to 1000 m of shales were deposited during the Givetian. The early Frasnian is marked by the ubiquitous sedimentation of black shales and bituminous styliolinites. These lithologies occur repeatedly already during the Middle Devonian and document intermittent anoxic conditions. The basin and ridge topography is levelled by the shallowing-up sequence of up to 1400 m thick upper Frasnian and Famennian shales which grade into a deltaic sequence of uppermost Famennian/Tournaisian sandstones. The up to now only vaguely discriminated lithostratigraphic formations of the Devonian have been biostratigraphically defined in suitable type sections.

An exhumed Palaeozoic underwater scenery: the Visean mud mounds of the eastern Anti-Atlas (Morocco)

About one hundred carbonate mud mounds, covering an area of 440 km 2 in the eastern Anti-Atlas of Morocco, constitute one of the largest mound agglomerations known so far from Lower Carboniferous settings. They occur within a 4000-m-thick succession of shales with intercalated bedded limestones, sandstones, and siltstones. According to conodont and goniatite biostratigraphy, mound formation started in the early Gnathodus texanus Zone and terminated during the G. bilineatus Zone of the Visean stage. Individual mounds are a few metres to 30 m high, have base diameters of up to 300 m and are concentrated in several parallel, WNW–ESE running belts. From their lithology and facies relationships, four types of mounds can be distinguished: (1) massive crinoidal wacke- or packstones without stromatactis; (2) massive crinoidal wacke- or packstones with rare stromatactis; (3) similar to (2), but allochthonous; and (4) biodetrital (skeletal) grainstone mounds. While carbonate deposition in types (1) to (3) was probably triggered by microbial precipitation, type (4) is the result of a predominantly mechanical accumulation of skeletal debris. Biota in the four types comprise a great variety of invertebrates, among which crinoids, sponges, and bryozoans are most common. Diagenesis of the mound carbonates was dominated by recrystallization of micritic matrix and organic remains and late burial cementation. Oxygen and carbon isotope data of brachiopod and crinoid ossicles, matrix, and early marine cements plot in a large field and do not allow definite conclusions about the composition of the ambient seawater. Microbial activity and the absence or scarcity of green algae, colonial corals and coralline sponges suggest deposition of the mounds in moderate water depth close to the lower limit of the photic zone. D 2001 Elsevier Science B.V. All rights reserved.

Diagenesis of Middle Devonian Carbonate Mounds of the Mader Basin (Eastern Anti-Atlas, Morocco)

ABSTRACT In the Mader Basin of the eastern Anti-Atlas, Middle Devonian carbonate mounds were established on a gently sloping ramp at moderate water depths. Primary stromatactis cavities in these mounds exhibit early marine, shallow burial, and deeper burial cementation. The mound matrix was affected by neomorphic alteration, stylolitization, and dolomitization. Earliest cementation occurred in the marine environment. Nonluminescent to blotchy-luminescent, cloudy radiaxial calcites of originally high Mg-calcite composition coat the walls of stromatactis cavities and take up more than 50% of the primary pore space. The isotopic signature (d18O = -2.6 (± 0.2) PDB; d13C = +2.7 (± 0.5) PDB) is close to that of nonluminescent, fabric-retentive brachiopod shells in the same mounds, and taken as the nearly pristine stable isotopic signature of Mader Basin seawater. The high d18O values, compared with Middle Devonian data of North America, are interpreted to result from the mid-latitudinal (ca. 35° S), temperate-water settings of the Mader Basin carbonate mounds. The transition to the shallow burial realm is indicated by a typical non-bright-dull luminescence sequence. Nonluminescent scalenohedral cements pass into bright-luminescent and banded-luminescent scalenohedral cements, which are overgrown in turn by dull- to moderate-luminescent blocky spar. Meteoric influence on that cement sequence can be excluded. Deeper burial diagenesis is represented by stylolitization, ferroan calcite cementation, and neomorphic alteration of the fine-grained mound carbonates. Pressure solution and/or outgassing of CO2 are possible ion sources for ferroan calcite precipitation in the remaining pore space. 18O depletion d18O = -6.8 to -8.9 PDB) of these cements can be calculated to elevated precipitation temperatures of 21-33.5°C and burial depths of 420-670 m. Microspar, making up the bulk of the mounds, was formed by a one-step transformation of a precursor calcitic micrite. Low d18O values (-6.9 (p 2.1) PDB), mottled luminescence, and high Fe concentrations indicate that this transformation also took place in the deeper burial realm.

Conodont-based quantitative biostratigraphy for the Eifelian of the eastern Anti-Atlas, Morocco

The measured ranges of 52 conodont taxa present in 7 Eifelian sections of the eastern Anti-Atlas in southern Morocco have been as- sembled by graphic correlation method into a chronostratigraphic framework. The Anti- Atlas regional composite developed in this study provides much higher stratigraphic res- olution than the traditionally used conodont zonation. It allows a subdivision of the Eifelian into 65.7 composite standard units, derived from the Jebel Ou Driss section, which was se- lected as the standard reference section. The framework appears to have a linear fit to time, giving the first image of the relative duration of the conodont zones in the upper Emsian to lower Givetian interval. The costatus, kockeli- anus, and Lower varcus chrons appear to have lasted more than two times longer than the partitus, australis, and/or hemiansatus chrons. An alternative upper Eifelian-lower Givetian conodont zonation with zones of more equal duration is proposed. Several conodont index taxa in the Anti-Atlas area display an almost synchronous appearance, independent of fa- cies conditions and shelf paleobathymetry. The analysis by graphic correlation reveals significant differences in rate of sediment accu- mulation between the Tafilalt Platform, which had condensed sedimentation (2.5 m/m.y.), and the Mader Basin, characterized by rates up to 40 m/m.y. area, the Devonian rocks offer excellent condi- tions for detailed biostratigraphical studies. Their depositional history reflects the process of disin- tegration of the northern margin of the Sahara cra- ton (Wendt, 1985, 1988). In contrast to the rather uniformly developed Upper Silurian and Lower Devonian sedimentary rocks, differential subsi- dence affected the area during Eifelian time, and thus a platform to basin topography of the shelf began to originate. This phase is biostratigraphi- cally relatively well constrained. Conodonts were found in many sections, providing a base for cor- relation of Eifelian rocks in the Anti-Atlas (Bul- tynck and Hollard, 1980; Bultynck and Jacobs, 1981; Bultynck, 1985, 1987, 1989, 1991). The fauna dominated by Polygnathus elements is lo- cally very abundant, but some stratigraphically important Eifelian index taxa are rare or absent, so that the position of some zonal boundaries (e.g., the base of the Eifelian) cannot be precisely rec- ognized. This is why Bultynck (1987) proposed two alternative conodont zonations for the upper Eifelian-lower Givetian interval. In order to provide a more accurate basis for correlation of Eifelian rocks and events in north- ern Africa, we used the graphic correlation method to develop a composite standard section based on conodont data from the eastern Anti-At- las. Our regional chronostratigraphic framework (Anti-Atlas regional composite) provides a much more linear biostratigraphic subdivision than any previously used conodont zonation. It offers, moreover, the best possible stratigraphic resolu- tion, useful for detailed analysis of the sedimen- tary evolution of the study area during the Middle Devonian. This contribution constitutes a first ap- proach in the construction of the Eifelian com- posite standard with worldwide applicability. In conventional biostratigraphy, fossil zones are used to achieve correlation. The graphic cor- relation technique, however, produces a frame- work that is founded on biostratigraphy, but inte- grates all the biostratigraphic events onto a composite standard. This method was developed by Shaw (1964), and was subsequently extended (e.g., Miller, 1977; Sweet, 1979; Edwards, 1984, 1989) to be applicable to nonpaleontologic and nonunique, but chronostratigraphically signifi- cant, events (including magnetic polarity data, key lithological marker beds, various geophysical log- type data, and stable isotope logs). It constitutes a powerful tool for the analysis of synchroneity or nonsynchroneity of geologic events. Integrated with sequence stratigraphy and sedimentological data, Shaws method of graphic correlation is one of the standard techniques applied in modern basin analysis (e.g., Neal et al., 1994). GEOLOGIC SETTING

Numerical calibration of the Devonian-Carboniferous boundary: Two new U-Pb isotope dilution–thermal ionization mass spectrometry single-zircon ages from Hasselbachtal (Sauerland, Germany)

The Hasselbachtal section (Sauerland, Germany) is an auxiliary global stratotype of the Devonian-Carboniferous boundary and one of the most important reference sections for the evolution of the latest Famennian to earliest Tournaisian pelagic fauna. Biostratigraphically well controlled altered volcanic ash layers (metabentonites) intercalated in the section afford a perfect opportunity for a numerical fixing of this important Paleozoic period boundary. We have performed U-Pb isotope dilution-thermal ionization mass spectrometry (ID-TIMS) analyses on (sub)microgram-sized single zircons and zircon fragments extracted from two metabentonites (beds 79 and 70) in the lowermost Tournaisian part of the section. Bed 79 metabentonite is positioned directly above the Devonian-Carboniferous boundary within the Siphonodella sulcata conodont zone. Five concordant analyses form a cluster with a 2 0 6 Pb/ 3 3 8 U concordia age of 360.5 ′ 0.8 Ma. Zircons of the next younger metabentonite (bed 70), in the lower Siphonodella duplicata conodont zone, yielded a tightly grouped cluster of 10 concordant analyses with a 2 0 6 Pb/ 2 3 8 U concordia age of 360.2 ′ 0.7 Ma. On the basis of these two new single-zircon ages and previously published late Famennian U-Pb ID-TIMS ages, the Devonian-Carboniferous boundary is reinterpolated herein to 360.7 ′ 0.7 Ma.

The Numerical Age of the Upper Frasnian (Upper Devonian) Kellwasser Horizons: A New U‐Pb Zircon Date from Steinbruch Schmidt (Kellerwald, Germany)

Steinbruch Schmidt (Kellerwald, Germany) is probably the world’s most famous locality that exposes the late Frasnian anoxic events (Kellwasser Crisis). A 3‐cm‐thick bentonite layer (Bed 36) is intercalated between the two Kellwasser horizons and can be precisely assigned to the Upper rhenana conodont Zone. Twenty‐four U‐Pb ID‐TIMS (isotope dilution–thermal ionization mass spectrometry) analyses of single zircons extracted from this layer yielded 17 concordant results, which form an elongated cluster along concordia. The oldest point of this cluster is assumed to represent the lowest amount of Pb loss, and its 206Pb/238U age of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Mud buildups on a Middle Devonian carbonate ramp (Algerian Sahara)

377.2\pm 1.7