Initiation and Development of Small-Scale Sponge Mud-Mounds, Late Jurassic, Southern Franconian Alb, Germany
- C. L. V. Monty,
- D. W. J. Bosence,
- P. H. Bridges and
- B. R. Pratt
Published Online: 14 APR 2009
Copyright © 1995 The International Association of Sedimentologists
Carbonate Mud-Mounds: Their Origin and Evolution
How to Cite
Hammes, U. (1995) Initiation and Development of Small-Scale Sponge Mud-Mounds, Late Jurassic, Southern Franconian Alb, Germany, in Carbonate Mud-Mounds: Their Origin and Evolution (eds C. L. V. Monty, D. W. J. Bosence, P. H. Bridges and B. R. Pratt), Blackwell Publishing Ltd., Oxford, UK. doi: 10.1002/9781444304114.ch11
- Published Online: 14 APR 2009
- Published Print: 17 JUL 1995
Print ISBN: 9780865429338
Online ISBN: 9781444304114
- development of sponge mud-mounds;
- Dedolomitization and meteoric cementation;
- Peloidal crusts along with sponges- main frame builders of mud-mounds;
- Oxfordian carbonate deposits to sea-level fluctuations to intermediate burial dolo-mitization in the sponge mud-mounds
The development of Late Jurassic (middle to late Oxfordian) sponge mud-mounds was studied at the western margin of the Southern Franconian Alb, Germany. Several embryonic sponge mud-mounds, intercalated with sponge-bearing calcareous marls, occur above a thin layer of early Oxfordian glauconitic marls. These scattered basal sponge mud-mounds gradually developed from biostrome-like sponge patches to larger, sponge-crust mud-mounds. The accumulation of micritic mud entrapped and/or precipitated by bacterial activity as well as by the baffling action of sponges built the rigid frame of these mounds.
Four distinctive facies characterize the temporal and spatial development of the mud-mounds. The sponge mud-mound basal facies consists of boundstone and wackestone to packstone fabrics in embryonic sponge mud-mounds (1 × 1.5 m), and sponge-bearing marls. The more massive (4 × 5 m) mud-mound facies is characterized by a basal spiculitic mudstone that is sharply overlain by sponge-bearing peloidal crust boundstones. The proximal flank facies consists of bedded peloid-intraclast packstones and small sponge mud-mounds (0.5 × 1 m) of sponge-crust boundstones. Finally, the thin-bedded distal flank facies is characterized by peloid-tuberoid (sponge and crust fragments) wackestone to mudstone fabrics. The diversity of organisms, density and median size of tuberoids, peloids, and intraclasts rapidly decrease within a lateral distance of 20 m from the mud-mound core.
The facies patterns outline a two-stage development of the sponge mud-mounds. Stage 1, the colonization and stabilization stage, is characterized by pioneer communities dominated by siliceous sponges inhabiting soft substrates accompanied by a highly diverse marine fauna. Where peloidal crusts bound the sediment and decaying sponges, embryonic mud-mounds and eventually biostromal sponge-patches developed. Slight sea-level fluctuations drowned the first stage, thus probably preventing further ecological diversification. Stage 2, the domination stage, is characterized by siliceous sponges and mainly peloidal crusts that build a rigid, compaction-resistant framework owing to synsedimentary lithification and sediment binding.
Diagenetic patterns reveal submarine cementation of all facies. No early freshwater influence was observed. Lithification on the sea-floor is evinced by boring and encrusting organisms, and by small submarine fissures. Two stages of dolomitization, preferentially in boundstones, occurred under shallow to intermediate burial conditions. Dedolomitization and meteoric cementation are recent and ongoing processes.