RESEARCH REPORTS 349 Effect of Extrinsic Factors on Biofabric and Brachiopod Alteration in a Shallow Intraplatform Carbonate Setting (Upper Triassic, West Carpathians) ADAM TOMASÆ OVY CH Institut fuÈ r PalaÈontologie, WuÈ rzburg UniversitaÈ t, Pleicherwall 1, 97070 WuÈ rzburg, Germany, Email: [email protected] PALAIOS, 2004, V. 19, p. 349±371 potential of brachiopods due to differential extrinsic factors (e.g., between hard- and soft-bottom settings) can substan- Upper Triassic assemblages containing the terebratulid tially bias the understanding their ecology and temporal brachiopod Rhaetina gregaria from a shallow, intraplat- shifts in environmental preferences. Data about substantial form carbonate setting of the Fatra Formation are classi®ed bioerosion/micritization of brachiopods in some deposit according to biofabric, geometry, and internal structure types indicate their higher durability and inherently higher into 6 deposit types, which are interpreted as: (1) autochtho- preservation potential in contrast to actualistic data about nous primary biogenic, (2) autochthonous winnowed or sed- the poor resistance of modern brachiopods to destruction. iment starved, (3) parautochthonous storm-wave, (4) par- autochthonous storm wave/¯ow, (5) amalgamated storm- reworked, and (6) allochthonous (long-term current/wave) INTRODUCTION deposits. Their distribution on the bed scale correlates with depth-related environmental gradients in regard to the po- In actualistic studies of death assemblages, quantita- sition of fair-weather wave base, average storm wave base, tive analyses based on taphonomic signatures (the mea- and maximum storm wave base. The biofabric, geometry, sures that detect intensity and magnitude of shell alter- and internal structure of brachiopod deposits were predom- ation) have been used to de®ne taphofacies types that are inantly in¯uenced by: (1) storm activity, related to varia- well correlated with environmental gradients (Powell et tions in sedimentation rates and water energy; and (2) orig- al., 1989; Meldahl and Flessa, 1990; Staff and Powell, inal variations in composition and spatial distribution of 1990; Kowalewski et al., 1994; Nebelsick, 1999). In addi- life associations. Fossil assemblages preserved in brachio- tion to taphonomic signatures, deposit-level properties re- pod deposits have a wide range of temporal resolution, lated to biofabric, geometry, and internal structure of de- ranging from census to environmentally condensed types. posits are used to de®ne taphofacies of fossil assemblages Brachiopod assemblages in the storm-reworked deposits (Brett and Baird, 1986; Speyer and Brett, 1988, 1991; probably were affected by catastrophic mortality. The dis- OloÂriz et al., 2002; Wani, 2003) because they help incor- tinction of brachiopod deposit types based on deposit-level porate effects of background and episodic processes, such criteria does not wholly correspond to the classi®cation of as bioturbation, long-term wave activity, or episodic storm taphofacies types based on intensity of shell alteration. The activity. The term shell concentration often is used in biofabric and associated deposit-level properties re¯ect ®nal taphonomic analyses of fossil assemblages (Kidwell et al., depositional processes (i.e., the rate and permanence of 1986). De®nitions of shell concentrations are similarly burial), whereas shell alteration of brachiopods re¯ects based on both their deposit-level properties (biofabric, ge- mainly variation in the nature of pre-burial environmental ometry, internal structure) and the taphonomic signa- conditions. The lowest degree of alteration (i.e., low levels of tures (Parsons et al., 1988; Kidwell, 1991a; FuÈ rsich and bioerosion, micritization, encrustation, and disarticulation) Oschmann, 1993; Abbot, 1997; SimoÄes and Kowalewski, is associated with deposits that were affected by storm-in- 1998; FuÈ rsich and Pandey, 1999; Mandic and Piller, 2001; duced sudden burial. In general, settings with high propor- Nebelsick and Kroh, 2002; Zuschin and Stanton, 2002). A tions of micritic mud (associated with mixed brachiopod-bi- comparative approach based on combining both of these valve associations) are characterized by relatively low alter- data types can be used to provide a high-resolution tool for ation of brachiopods. These settings are in sharp contrast to interpretation of paleoenvironment. hard-bottom settings (associated with coral associations), Although there is a lot of actualistic information about in which bioerosion and micritization are high. This differ- the variation of taphonomic signatures of molluscs (see ence in shell alteration is the effect of extrinsic factors relat- Kidwell et al. 2001), there are few actualistic studies con- ed to lower turbidity, higher proportion of hardparts and cerning alteration of articulate brachiopods across envi- higher storm reworking in latter settings. ronmental boundaries. Because articulate brachiopods Autochthonous/parautochthonous benthic associations are characterized by a unique shell structure and compo- dominated by the short-looped terebratulid Rhaetina gre- sition, further study concerning their alteration patterns garia are typical of settings below the fair-weather wave and preservation potential is needed. Some taphonomic base, with background low-energy condition. This is in con- signatures are relatively straightforward. For example, an trast to high-energy/hard-bottom occurrences of this asso- abundance of articulated shells nearly excludes the possi- ciation from other regions. The difference in preservation bility of long-term mechanical reworking. However, shell Copyright Q 2004, SEPM (Society for Sedimentary Geology) 0883-1351/04/0019-0349/$3.00 350 TOMASÆ OVY CH resistance to disarticulation when exposed on the sea¯oor, as well as the role of organic-matrix decay on rates of dis- articulation, are poorly known (e.g., Daley, 1993). Rather than use taphonomic signatures as unambiguous evidence of environmental conditions in taphofacies analyses of fos- sil assemblages, their resolution potential for paleoenvi- ronmental interpretation should be tested independently in taphonomic analyses. Therefore, these two types of taphonomic data (deposit- level properties and taphonomic signatures), potentially re¯ecting different processes operating at different scales (Davies et al., 1989a, b; FuÈ rsich, 1995; Behrensmeyer et al., 2000), are analyzed separately in this study of brachio- pod assemblages. Two speci®c aspects are addressed here: (1) the variation in biofabric (i.e., three-dimensional ar- rangement of skeletal elements), geometry, and internal structure of brachiopod deposits with respect to environ- mental gradients; and (2) the effect of extrinsic environ- mental factors on brachiopod shell alteration. The ®rst as- pect provides the basic framework for genetic interpreta- tion of brachiopod deposits. The second aspect provides de- tailed insights into taphonomic pathways (i.e., the rates, selectivity, and importance of particular destructive and constructive processes) during the formation of death as- semblage (Meldahl and Flessa, 1990; Kowalewski et al., 1994; Macchioni, 2000; Wani, 2001). This aspect is very important because it enables testing of differences in pres- ervation potential of brachiopod associations among set- tings and addressing of questions related to its composi- tional ®delity (i.e., the quantitative faithfulness of the re- cord of population-community-level features to the origi- nal biological signal; Behrensmeyer et al., 2001). In addition to addressing the role of extrinsic factors in alter- ation and preservation potential of brachiopod associa- tions at this local scale, this taphonomic analysis provides new insights into the relatively poorly known preservation potential of brachiopods, information that is relevant to the interpretation of brachiopod distribution patterns at all scales. In the ®rst part of this work, deposit-level properties are analyzed and interpreted in terms of environmental gra- dients. In the second part, the variation of taphonomic sig- natures is evaluated with respect to: (1) previously de®ned brachiopod deposit types, (2) benthic association types, and (3) deposit types with different packing density. Cor- relation of taphofacies types based on taphonomic signa- tures with the classi®cation of brachiopod deposit types based on their deposit-level properties also is examined. GEOLOGIC SETTING Paleogeography During the Upper Triassic, the West Carpathians were situated on the extensive epeiric carbonate platform on the northwestern margin of the Tethys Ocean in the sub- tropical climatic belt (Fig. 1A). The Rhaetian Fatra For- mation, which displays considerable facies variation both horizontally and vertically, was deposited in the shallow- FIGURE 1ÐPaleogeographic and geographic maps of the study area. water, intraplatform, marine, predominantly carbonate (A) General paleogeographic position of the Fatric Unit (Central West setting of the Fatric Unit (Central West Carpathians; Carpathians) in the Upper Triassic (Norian/Rhaetian; modi®ed after MichalõÂk (1994). (B) Regional map with location of the study area. (C) MichalõÂk, 1982; Fig. 2). Carbonate deposition is character- Geographic location of sections in the Vel'ka Fatra Mts. 1ÐDedosÆova; ized by reduced terrigenous input and low subsidence re- 2ÐMaly Zvolen; 3ÐBorisÆov; 4ÐBystry potok; 5ÐRaÂztoky; 6ÐKrõÂzna; 7ÐBelianska. TAPHONOMY OF UPPER TRIASSIC BRACHIOPOD DEPOSITS