JOURNAL OF SEDIMENTARY RESEARCH,VOL. 70, NO. 1, JANUARY, 2000, P. 170–180 Copyright  2000, SEPM (Society for Sedimentary Geology) 1073-130X/00/070-170/$03.00 THE SIGNIFICANCE OF HIATUS BEDS IN SHALLOW-WATER MUDSTONES: AN EXAMPLE FROM THE MIDDLE JURASSIC OF SWITZERLAND ANDREAS WETZEL AND VINCENZO ALLIA Geologisch-Pala ¨ontologisches Institut, Universita ¨t Basel, Bernoullistrasse 32, CH-4056 Basel, Switzerland e-mail: wetzel@ubaclu.unibas.ch FIG. 1.—Location map of the study area in northern Switzerland and southwestern Germany (upper part), and the sections studied in detail (lower part). ABSTRACT: Limestone beds formed in nearly carbonate-free shallow-water mudstones by discontinuous sedimentation and erosion are called hiatus beds. Anaerobic oxidation of organic matter by microbes provided excess alkalinity, inducing carbonate precipitation. A multiphase history of such beds is docu- mented from the Swiss Jurassic by several cementation and dissolution phases. Four cement types occur: micrite as earliest cement ( 13 C -10 to -20‰), stellate calcite between micrite-cemented parts ( 13 C -5 to -10‰), fibrous calcite cement in dissolution cavities ( 13 C -30‰), and blocky calcite in remaining pores ( 13 C -5‰). Except for the late blocky cement, all cements contain pyrite, indicating carbonate precipitation within the sulfate reduction zone. After early cementation by micrite, the beds moved relatively upwards into a shallower geochemical zone and some even to the seafloor because of erosion. Cavities formed during reburial by dissolution in the upper part of the sulfate reduction zone and in the upper part of the methanogenic zone. Strongly reduced sedimentation rates provided the requisite stable geochemical condi- tions for at least several thousands of years, which permitted precipitation and dissolution of carbonate by biochemical processes and diffusion. This happened on short-lived swells caused by differential subsidence and rotation along listric faults when basement structures became reactivated during the extensional stress regime from opening of the Tethys. During the Jurassic and Cretaceous breakup of Pangea the shelf area increased, and differential subsidence on these newly formed shelves was the main factor responsible for the observed post- Paleozoic maximum in hiatus beds and hiatus concretions. INTRODUCTION In predominantly fine-grained, siliciclastic sediments a break in sedimentation or seafloor erosion may be marked by a horizon of early diagenetic concretions, which have been referred to as ‘‘hiatus concretions’’ (Voigt 1968). In addition, certain limestone beds may form in this way (Raiswell 1988), which we suggest can be called ‘‘hiatus beds’’. Hiatus concretions show evidence that they hardened before exhumation, and cementation probably occurred not very deep below the sediment surface (e.g., Savrda and Bottjer 1988). It is considered a prerequisite for the growth of concretions that they reside for a considerable time ( 7000 years; Coleman and Raiswell 1993) within a specific geochemical environment, e.g., the sulfate reduction zone. Therefore, a significant interruption of sediment input may be sufficient to induce concretion formation (Spears 1989; Raiswell 1987), whereas seafloor erosion is not necessarily required. Disconformities in mudstones are inconspicuous if re- worked concretions or hiatus beds are lacking (Fu ¨rsich and Baird 1975; Baird 1976). Hiatus beds, thus, have stratigraphic and sedimentologic value in that they allow identification of surfaces at which sedimentation was interrupted for a significant time. Such discontinuity surfaces are commonly not manifested as biostratigraphic gaps (e.g., Wilson 1985). If concretions are exhumed, they form hard bottoms and the epifauna changes in response to substrate consistency (e.g., Fu ¨rsich 1979). Study of individual hiatus concretions and beds (e.g., Voigt 1968; Hallam 1969; Fu ¨rsich 1979) has revealed their complex, multiphase history; this includes (1) con- cretion formation; (2) concretion exhumation, (3) boring and/or encrustation; (4) burial; and (5) precipitation of additional cement. During phases 1 and 5 cracking and precipitation of additional cement may occur (e.g., Kennedy et al. 1977; Hes- selbo and Palmer 1992). In terms of sequence stratigraphy, hiatus concretions and beds are genetically linked to rising or high sealevel (e.g., Van Wagoner et al. 1988). They occur at the initiation of transgressions (e.g., Voigt 1968; Fu ¨rsich et al. 1991). Furthermore, during the time of maximum rate of transgression they form in areas where the sediment input is strongly reduced (‘‘condensed section’’ while clastics accumulate nearer to the land (e.g., Loutit et al. 1988). Similar deposits, thought to have formed during sealevel highstand, were also reported from drowned carbonate platforms by Kendall and Schlager (1981). This simple sequence-stratigraphic interpretation of hiatus beds and condensed sections, however, is not always valid. Other processes, such as differential subsi- dence, can produce sediment starvation or seafloor erosion (e.g., Hesselbo and Palm- er 1992) and thus provide the conditions necessary for formation of hiatus concre- tions. It is the purpose of this study to demonstrate that hiatus beds in the Swiss Jurassic are useful for deciphering the effects of differential subsidence on the deposition of a thick mudstone sequence that accumulated rapidly in an epicontinental basin at a time of minor sealevel changes. The duration and type of discontinuous sediment input is reflected by the overall setting of the hiatus beds and the various cements therein. In general, hiatus beds seem to be formed more frequently during times of tectonic activity than of intense sealevel changes. STUDY AREA AND GEOLOGICAL BACKGROUND Several sections of Middle Jurassic mudstones were studied in northern Switzer- land and southwestern Germany (Fig. 1). Because they accumulated mainly during