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Limestone-Marl Alternations in Epeiric Sea Settings – Witnesses of Environmental Changes, Or of Rhythmic Diagenesis?

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Limestone-Marl Alternations in Epeiric Sea Settings – Witnesses of Environmental Changes, Or of Rhythmic Diagenesis? Limestone-marl alternations in epeiric sea settings – witnesses of environmental changes, or of rhythmic diagenesis? Hildegard Westphal 1, Axel Munnecke 2, Florian Böhm 3, Stefan Bornholdt 4 1) Fachbereich Geowissenschaften, Universität Bremen, Marum Building, Leobener Straße, D-28359 Bremen, Germany, [email protected] 2) Institut für Paläontologie, Universität Erlangen, Loewenichstraße 28, D-91054 Erlangen, Germany, [email protected] 3) Leibniz-Institut für Meereswissenschaften, IfM-GEOMAR, Wischhofstraße 1-3, D- 24114 Kiel, Germany, [email protected] 4) Institut für Theoretische Physik, Universität Bremen, Otto-Hahn-Allee, D-28359 Bremen, Germany, [email protected] For: Dynamics of Epeiric Seas: Sedimentological, Paleontological and Geochemical Perspectives ; edited by Chris Holmden and Brian R. Pratt; Geological Association of Canada Special Volume “[A test] appears to indicate that many limestones can be produced solely by rhythmic unmixing of CaCO 3 during diagenesis. Unless such limestones can be clearly distinguished from those that record genuine environmental signals, orbital cycle analysis based on such sequences will give meaningless results“(Hallam, 1986) Westphal et al. “ Limestone-marl alternations in epeiric sea settings ” 2 Abstract absolute concentrations). Systematic differences in diagenetically inert parameters can provide Limestone-marl alternations are widespread and unequivocal proof of primary differences. In the typical sediments of epeiric basins and are present in studied limestone-marl alternations, however, such variable abundance throughout the entire parameters do not directly reflect the lithological Phanerozoic. In many cases, their rhythmic rhythm, shedding doubt on limestone-marl appearance is interpreted as a direct response to alternations as direct archives of environmental orbital forcing. However, it is a challenge to change. unequivocally prove a sedimentary origin of the rhythmic intercalation of the two lithologies. This Box model computer simulations visualize difficulty arises from differential diagenesis that possible effects of early diagenetic change acting on alters limestone beds in different ways than limestone-marl alternations, independent of the interlayers (marls), causing a loss of comparability presence or absence of a primary rhythm. The between the lithologies. Differential diagenesis, simulations demonstrate that diagenesis has the between other effects, causes passive enrichment of potential to seriously distort any primary rhythm Fig. 1: Occurrence of fine-grained calcareous rhythmites (limestone-marl alternations and nodular limestone successions) from literature compilation based on databases including ISI Web of Science, GEOREF and GEOBASE (data compilation available upon request from the authors). The reported occurrences are normalized for the time span of the interval they represent. Also shown: tropical shelf area (TSA) through time (dashed line, after Walker et al., 2002), and ratio of Mg to Ca ions in ocean waters through time (solid line, after Stanley and Hardie, 1998). the inert non-carbonate fraction in interlayers, where present in the pristine sediment. In particular, calcium carbonate is being dissolved, as well as differential compaction acting mainly on the marl passive dilution in limestone beds, that are cemented interlayers induces distortions of the ratios of the by imported calcium carbonate. Therefore, original frequencies. These simulations emphasize unequivocal information about systematic differences the difficulties in conducting frequency analyses on in the precursor sediments of limestones and carbonate contents of real-world successions. interlayers therefore is preserved only in parameters that are not modified during diagenesis. Such Keywords for index: diagenetically inert parameters include the spectra of anactualism, aragonite, aragonite sea, calcite, calcite organic microfossils (but not their absolute sea, cellular automaton, cementation, compaction, concentration in the bulk sediment) and the ratios of computer simulation, diagenesis, differential diagenetically inert trace elements (again not the Westphal et al. “ Limestone-marl alternations in epeiric sea settings ” 3 diagenesis, dissolution, distortion, environmental more distal parts of the basin. (2) A shallow wave archive, environmental conditions, insolubles, base, as usually encountered in epeiric seas, and the limestone-marl alternation, Milankovitch, model, low relief prevent deposition of coarse-grained overprint, palynomorphs, sedimentary record, self- sediments in the distal parts of the basin and favor the organization, stratigraphic record, rhythmites fine-grained sedimentation typical for limestone-marl altenations. (3) Furthermore, the surrounding land provides the input of siliciclastics, of which, in a 1. Introduction – a brief review of limestone- sufficiently large epeiric basin, mostly clay-sized marl alternations material will reach the depositional site where it can form marls. Such an epeiric setting provides all ingredients Limestone-marl alternations are a widespread and for forming potentially sensitive recorders of climate characteristic facies of epeiric sea basins with famous cycles as fine-grained calcareous rhythmites. examples in the Ordovician of N-America and the Therefore, the cyclic patterns of fine-grained western and central High Atlas (Morocco), the calcareous rhythmites are often regarded as indicators Silurian of Northern Europe, the Triassic of the and recorders of orbitally forced climatic cycles (see Carpathians, the Mississippian of Montana, the chapters in de Boer and Smith, 1994 and Einsele and Jurassic and Cretaceous of Central and Western Ricken, 1991). In contrast to shallow-water carbonate Europe, and the Cretaceous of the Western Interior platfoms, the basinal setting allows for continuous Seaway and in Venezuela (e.g., Davaud and records even during sea-level lowstands. Lombard, 1975; Courtinat, 1993; Holmden et al., Nonetheless, the basinal sediments record the 1998; Samtleben et al., 2000; Westphal and environmental changes affecting the shallow-water Munnecke, 2003; Chacrone et al., 2004; Rey et al., platforms in their imported, shallow-water produced 2004; Tomasovytch, 2004; see also several chapters portion. The clay portion also potentially reflects in Einsele et al., 1991). Limestone-marl alternations climatic conditions that influence the weathering of are known from deposits of all Phanerozoic ages, the hinterland. In addition, varying nutrient fluxes even though their abundance varies strongly for the related to variations in weathering patterns are different geologic periods (Fig. 1; see also Westphal potentially recorded in paleontologic parameters such and Munnecke, 2003). The abundance roughly as calcareous nannofossil ratios. Thus, fine-grained follows the oscillations between calcite and aragonite calcareous rhythmites offer a wealth of parameters seas (Sandberg, 1983; Stanley and Hardie 1999) with that turn them into potential archives of climate high abundances during times of calcite seas and variations. lower abundances during times of aragonite seas. In Jurassic and Cretaceous epeiric sea deposits, such One difficulty, however, arises from the fact alternations are particularly widespread. that fine-grained calcareous rhythmites generally are strongly affected by differential diagenesis. Their Limestone-marl alternations are bimodal lithologic character of limestone beds characterized by their conspicuous outcrop intercalated in softer interlayers is largely a product appearance with a pronounced ABAB rhythm of of differential diagenesis, regardless of the presence more weathering-resistant limestone beds and softer or absence of primary sedimentary rhythms (see interbeds (see overview in Einsele and Ricken, 1991). below). This fact is still underestimated in many They can be viewed as part of a continuum of studies that interpret rhythms of various lithological, bimodal micritic alternations ranging from limestone- paleontological, or geochemical parameters in chalk to limestone-shale alternations, and including limestone-marl alternations as direct expressions of lithographic and nodular limestones, and well-bedded orbital climate forcing (cf. Fischer, 1980; Sprenger limestones (Munnecke and Samtleben, 1996; and ten Kate, 1993; Schwarzacher, 2000; Cleaveland Westphal et al., 2000; Munnecke et al., 2001; et al., 2002; Strasser et al. 2005). In this paper we Munnecke and Westphal, 2004). This group of will review and present published and new sedimentary facies is referred to as "fine-grained geochemical data and modeling results to illustrate calcareous rhythmites" in the present manuscript. the problems involved in interpreting fine-grained Large epeiric seas offer favourable conditions calcareous rhythmites as one-to-one recorders of for the generation of fine-grained calcareous climate cycles. rhythmites for several reasons (cf. various chapters in Einsele et al., 1991). (1) Tropical epeiric seas provide marginal areas for the production of shallow-water carbonate deposits that can be winnowed into the Westphal et al. “ Limestone-marl alternations in epeiric sea settings ” 4 1.1. What is so special about limestone- limestone beds was sourced from calcium carbonate marl alternations? dissolution in the interlayers (“donor” and “receptor” limestones of Bathurst [1971]; see also, e.g. , Ricken Fine-grained calcareous rhythmites show a wide 1986, 1987). Many
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