Reinhold R. Leinfelder, Stuttgart & Helmut Keupp, Berlin

Upper Jurassic mudmounds: Allochthonous sedimentation versus autochthonous carbonate production

In: Upper Jurassic mudmounds: Allochthonous sedimentation versus autochthonous carbonate production.- Neuweiler, F. & Reitner, J. (coord.), Mudmounds: A polygenetic spectrum of fine-grained carbonate buildups.-

Facies, 32, 17-26, Erlangen, 1995.

Note: This is Part III of a larger paper on mudmounds (Neuweiler & Reitner, coord.) and as such does not have a separate abstract. Below, the conclusions of the Upper Jurassic mundmound chapter are given. You may also see contents, keywords and summary of the entire paper

Conclusions of Part III (Upper Jurassic mudmounds)

• Many Upper Jurassic buildups represent mudmounds. They thus are dominated by cryptocrystalline material, a variable amount of which represents primarily soft mud. Macrofauna occurs in variable quantities but does not form a rigid framework. Most mounds represent siliceous sponge - microbial crust mudmounds, but coral mudmounds occur occasionally.
• Hard automicrites, most of which are of the crust-type, were important in the formation of mudmounds, by contributing to the mound material and stabilising soft areas. In comparison with modern counterparts (cf. Reitner 1993) their microfabrics suggest that they mostly or entirely represent precipitates within the extracellulary polymeric substance of organic films or mats largely produced by microbial activity.
• Bioturbation, the existence of gravitational debrites composed of crust-fragment and other features indicate the existence of soft mud which includes both allomicrites and mud produced in-place within the mound by a combination of biodestruction and bioproduction. Soft in-place mud must be assumed for bioturbated, micritic, crust-rich carbonate mudmound developing within clay-dominated successions. Soft mud, both in-place and allochthonous, is thought to represent a major constitutent of Oxfordian mounds but may become less abundant towards the Late Kimmeridgian.
• Low abundance of calcareous crusts in some mounds or parts of mounds, together with features indicating soft mud suggest that uncalcified microbial films and mats also played a major role in stabilising soft sediment areas as well as trapping allomicrites. Early cementation of soft mud is obvious in transported mudmound blocks, and may have commenced within microbial mats. As obvious from a comparison with supratidal Jurassic loferites, mats which trapped particles do not show a distinct precipitation fabric although they may be hardened very early. This is in contrast with the microbial crusts which formed under zero background sedimentation and show distinct features which make them discernable from the overlying and underlying micritic sediments.
• Upper Jurassic mudmounds largely developed during third-order TST and early HST, when allochthonous sedimentation was strongly reduced. It is assumed that allochthonous material was imported during storms or high-frequency regressions, and swept towards the sticky surfaces of the mounds by contour currents. Phases of very reduced or zero sedimentation allowed for the development of microbial crust and other automicrites and facilitated early diagenetic hardening at the sea floor.
• Some Upper Jurassic mudmounds grew directly below fair weather wave base (coral mounds), although the majority of them grew in homoclinal to near level-bottom mid to upper outer ramp settings. The latter characteristically contain siliceous sponges in very variable quantities. Rare phases of oxygen depletion occasionally prevented the growth of macroorganisms.
• Future research on Jurassic mudmounds will have to focus on better identification of automicrites versus allomicrites and on the detection of features indicative of primary hard versus soft material as well as the detection of early diagenetic hardening.

Mud Mounds: A Polygenetic Spectrum of Fine-grained Carbonate Buildups

Joachim Reitner and Fritz Neuweiler, Göttingen (Coordination)

Gerd Flajs, Manfred Vigener, Aachen; Helmut Keupp, Berlin; Dieter Meischner, Fritz Neuweiler, Josef Paul, Joachim Reitner, Klaus Warnke, Göttingen; Helmut Weller, Greifswald; Patrick Dingle, Christian Hensen, Priska Schäfer, Kiel; Pascale Gautret, Paris; Reinhold R. Leinfelder, Stuttgart; Hansmartin Hüssner, Bernd Kaufmann, Tübingen

FACIES, 32, 1-70, ERLANGEN 1995

KEYWORDS: MUD MOUNDS - SPONGES - AUTOMICRlTES - ORGANOMICRITES STROMATACTIS - PELOIDS - THROMBOLITES - MICROBIAL COM MUNl~IES - BENTHIC COMMUNlTlES - BIOFILMS - MICROBIALlTES MACROMOLECULES -REEFEVOLUTION-DEVONIAN-CARBONIFEROUS - PERMIAN - TRIASSIC - CRETACEOUS - HOLOCENE

CONTENTS

• Summary
• Part I: Mud Mounds: Recognizing a polygenetic spectrum of fine-grained carbonate buildups.(J.REITNER&F.NEUWEILER)
• Part II: Modern and fossil automicrites: Implications for mudmound genesis. (J. REITNER, F. NEUWEILER & P. GAUTRET)
• Part III: Upper Jurassic mud mounds: Allochthonous sedimentation versus autochthonous carbonate production. (R.R. LEINFELDER & H. KEUPP)
• Part IV: Epifluorescence-microscopy of selected automicrites from Lower Carnian Cipit-boulders of the Cassian Formation (Seeland Alpe, Dolomites). (F. NEUWEILER & J. REITNER)
• Part V: Stromatolite reefs of the Upper Permian Zechstein Basin (Central Europe). (J. PAUL)
• Part VI: Primary and diagenetic mud mound genesis in the San Emiliano Formation of the Carmenes Syncline (Westphalian B/C, Cantabrian Mts., N. Spain). (C. HENSEN, P. DINGLE & P. SCHÄFER)
• Part VII: Origin and depositional environment of Lower Carboniferous mudmounds of Northwestern lreland.(K.WARNKE & D. MEISCHNER)
• Part VIII The Devonian mud mound of Rübeland in the Harz Mountains/Germany. (H. WELLER)
• Part IX: Middle Devonian mud mounds of the Ma' der Basin in the eastem Anti-Atlas, Morocco. (B. KAUFMANN)
• Part X: Lower Devonian stromatactis mud mounds (Montagne Noire, France). (G. FLAIS, H. HÜSSNER & M. VIGENER).
• Part XI: Supposed principal controlling factors of rigid micrite buildups. (J. REITNER & F. NEUWEILER)
• References

Summary of entire paper

This 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 of in situ forrned micrites (automicrites) in close association with siliceous sponges is documented in Devonian, Carboniferous, Triaissic, 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 com prise: (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 precur sor 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.