Growth Dynamics and Ecology of Upper Jurassic Mounds, with Comparisons to Mid-Palaeozoic Mounds
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Sedimentary Geology 145 (2001) 343–376 www.elsevier.com/locate/sedgeo Growth dynamics and ecology of Upper Jurassic mounds, with comparisons to Mid-Palaeozoic mounds Dieter U. Schmida,*, Reinhold R. Leinfeldera, Martin Noseb aInstitut fu¨r Pala¨ontologie und Historische Geologie der Ludwig-Maximilians, Ludwig-Maximilians Universita¨t, Richard-Wagner-Str. 10, D-80333 Munich, Germany bBayerische Staatssammlung fu¨r Pala¨ontologie und Geologie, Richard-Wagner-Str. 10, D-80333 Munich, Germany Received 8 June 2000; accepted 15 May 2001 Abstract The Mid-Palaeozoic, including the Late Jurassic, was a time of both widespread coral reef growth and pronounced mound formation. A comparison of mound features and their general setting highlights, despite all differences, general similarities in overall growth dynamics. Mound formation was frequently driven by discontinuous patterns, particularly by background sedimentation. In many examples, episodes of mound stabilisation by early lithification, growth of microbolite crusts and winnowing of fines was followed by growth episodes of benthic fauna under reduced to negligible background sedimentation. This pattern of variable sedimentation and organic buildups may have occurred in different orders and magnitudes, inducing a fractal pattern in some mound complexes. A composite approach in estimating growth rates of mounds demonstrates that high- frequency oscillations necessary for growth of most mounds might have ranged from a few thousand years to 4th and 5th order Milankovich cycles that were superimposed by autocyclic factors. D 2001 Elsevier Science B.V. All rights reserved. Keywords: Mounds; Jurassic; Mid-Palaeozoic; Sedimentation; Ramps; Stromatactis 1. Introduction 1992). During the latter episodes, mounds encom- passed a great variety of types, some even displaying Biologically induced carbonate mound structures characteristics transitional into shallow-water frame- rich in micrite are a common element throughout work reefs. The general palaeogeographic settings of major parts of earth history and occur in a great time intervals rich in both mounds and reefs are variety of subtypes and variable environmental set- comparable to a large degree. Global sea-level was tings. Some long intervals of geological time were mostly rising or relatively high, which resulted in dominated by mounds, with true framework reefs wide tropical–subtropical shelf areas and associated lacking (Early and Late Palaeozoic), while in other epeiric seas. Tropical carbonate shelf morphologies geological intervals (e.g., Mid-Palaeozoic and Late were largely dominated by ramps, some modified by Jurassic), mounds frequently co-occurred with shal- local or regional tectonics into steepened ramps or low-water framework reefs (cf. James and Bourque, even rimmed shelves. Climate was generally equili- brated, possibly resulting in reduced water circulation, * Corresponding author. Fax: +49-89-2180-6601. with episodes of oxygen depletion (expressions of E-mail address: [email protected] (D.U. Schmid). which are the common black shales and bituminous 0037-0738/01/$ - see front matter D 2001 Elsevier Science B.V. All rights reserved. PII: S 0037-0738(01)00157-9 344 D.U. Schmid et al. / Sedimentary Geology 145 (2001) 343–376 limestone intervals formed during these times; Lein- Australia (Wood, 1999) and from Poland (Holy Cross felder et al., 1994). Mountains; e.g., Racki, 1992). During the Late Jurassic, mounds were especially Mid-Palaeozoic mounds generally developed as common along the northern Tethys margin and its stromatactis- and cement-rich horizons with variable adjacent shelf seas. These complexes have been the metazoan content (e.g., bryozoans, corals, stromatop- subject of many papers, most focusing on sponge– oroids, crinoids). Although mostly initiated in deeper microbolite mud-mounds that developed within deeper water, many of these mounds display shallowing- ramp settings. A wide Upper Jurassic sponge mound upward successions. The latest stages developed in belt can be traced across Europe from southern Portu- fairly shallow water (see below; cf. Wood, 1999). gal and southeastern Spain, through southwestern In this study, we present an overview of the great France, northern Switzerland, and southern Germany, variety of mound types existing in the Jurassic, and give into central Poland and Romania. In Germany, these a first brief outline of representative Mid-Palaeozoic well-documented mounds represent the ‘classic’ exam- mound types. We also discuss the controlling mecha- ples of sponge–microbial reefs (Wagenplast, 1972; nisms leading to the development of different mound Gwinner, 1976; Flu¨gel and Steiger, 1981). Such types in shallow and deeper-water settings, and compare deeper-water siliceous sponge-bearing microbial the Jurassic and Mid-Palaeozoic examples in order to mud-mounds are also reported from the subsurface get a better understanding of the general processes and off eastern North America (Nova Scotia; Eliuk, 1978; prerequisites involved in mound formation. Jansa et al., 1982) and other non-European regions such as Caucasia (Kuznetsov, 1993) and Argentina 1.1. Terminology and definitions (Legarreta, 1991). They also appear in the Lower Jurassic of Portugal (Duarte et al., 2000), Morocco 1.1.1. Mounds (du Dresnay et al., 1978), and in the Middle Jurassic of According to James and Bourque (1992), mounds Spain (Giner and Barnolas, 1979). Besides these can be subdivided into three categories, which we, to deeper-water ‘classical’ mound occurrences, there are a large extent, adopt in this paper: (1) microbial several examples of mound development in shallow- mounds, dominated by peloidal or dense micrite water environments (e.g., Upper Jurassic of Portugal produced by calcifying benthic microbial associations and Spain). These complexes are generally composed (thrombolites, stromatolites, leiolites); (2) skeletal of scleractinian corals, stromatoporoids, detrital mud mounds, in which skeletal metazoans (e.g., corals, and minor amounts of binding microbial crusts (Nose, sponges) are largely prevailing; and (3) mud-mounds, 1995). predominantly composed of detrital mud (Fig. 1). During Mid-Palaeozoic times (Ordovician–Late There is still ongoing debate as to what actually Devonian), mounds developed mainly in the Balto- constitutes an ancient reef or mound, resulting in scandian region (e.g., Upper Ordovician of Lake many different definitions and review articles about Siljan), in eastern North America (e.g., Ordovician the terms reef, mound, buildup, etc. (Heckel, 1974; of Western Newfoundland; Pohler and James, 1989; Wilson, 1975; James, 1983, 1984; James and Bour- Stenzel and James, 1995), and in the Canadian Arctic que, 1992; Bosence and Bridges, 1995, and others). and Michigan Basin regions (Silurian; Narbonne and We define the term ‘‘reef’’ in a broad, pragmatic way, Dixon, 1989; Pratt, 1995). Silurian mounds also occur as encompassing all carbonate structures with orga- on the Gaspe´ Peninsula, Que´bec (e.g., Bourque and nic control during growth and a laterally restricted Gignac, 1984). Devonian mounds are reported from extension, which may have developed a weak or the Canadian Rocky Mountains (Alberta; Pratt, 1995), pronounced positive relief (cf. Tucker and Wright, Northern Africa (Algeria and Morocco; Wendt, 1993; 1990; Flu¨gel and Flu¨gel-Kahler, 1992). In contrast to Wendt et al., 1997), the Rhenohercynian belt in west- some definitions which differentiate between reefs and ern and central Europe (Belgium, Rhenish Slate mounds in terms of hydrodynamically stable organic Mountains, Harz Mountains; Burchette, 1981; Struve, framework formation (James and Bourque, 1992), we 1986; Bourque and Boulvain, 1993; Weller, 1995; characterise mounds as a subtype of reefs, representing Boulvain and Coen-Aubert, 1997; Pohler et al., 1999), lenticular or knobby bodies, which mainly consisted of D.U. Schmid et al. / Sedimentary Geology 145 (2001) 343-376 345 et al., 1995) that was indurated soon after its for- mation. In this paper, mounds are defined as struc- tures containing more than 25% of detrital or microbially generated structureless micrite. These structures developed in both deep and shallow waters, generally low-energy environments such as deep basins, lower slopes, slope/shelf breaks, intra-shelf areas and lagoons. Our definition of mounds follows several review articles (James, 1983, 1984), and its usage has continued in recent textbooks (Tucker and Wright, 1990). 1.1.2. Stromatactis There is a long-lasting discussion about the defi- Fig. 1. Process related compositional classification of mound types nition and genesis of stromatactis, which remain enig- used in this paper. matic structures. Most authors agree upon the following essential criteria for defining stromatactis: sparitic structures embedded in micrite-dominated dense micritic carbonate with varying amounts of carbonate; flat to undulose smooth lower surface; bioclasts, organic bindstones and benthic metazoans. digitate upper surface; internal sediment present; spar- The micritic carbonate might have been originally itic areas form a three-dimensional network (e.g., deposited as soft mottled mud, or it might have been Bourque and Boulvain, 1993). There is, however, no microbially precipitated carbonate (‘leiolites’; Braga real consensus about the inclusion of early diagenetic Fig. 2. Location map of investigated mound sites in Europe (a) and in SE Canada (b). 346 D.U. Schmid et al. / Sedimentary Geology 145 (2001)