Effect of Extrinsic Factors on Biofabric and Brachiopod Alteration in a Shallow Intraplatform Carbonate Setting (Upper Triassic, West Carpathians)

RESEARCH REPORTS 349

ADAM TOMAS˘ OVY´ CH Institut fu¨ r Pala¨ontologie, Wu¨ rzburg Universita¨ t, Pleicherwall 1, 97070 Wu¨ 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 classified 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) parautochthonous storm wave/flow, (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 define taphofacies types that are inantly influenced 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 define 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- Olo´riz 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 classification 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 reflect final taphonomic analyses of fossil assemblages (Kidwell et al., depositional processes (i.e., the rate and permanence of 1986). Definitions of shell concentrations are similarly burial), whereas shell alteration of brachiopods reflects 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; Fu¨ rsich and bioerosion, micritization, encrustation, and disarticulation) Oschmann, 1993; Abbot, 1997; Simo˜es and Kowalewski, is associated with deposits that were affected by storm-in- 1998; Fu¨ 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 ᮊ 2004, SEPM (Society for Sedimentary Geology) 0883-1351/04/0019-0349/$3.00 350 TOMASÆ OVYÂ CH resistance to disarticulation when exposed on the seafloor, as well as the role of organic-matrix decay on rates of disarticulation, are poorly known (e.g., Daley, 1993). Rather than use taphonomic signatures as unambiguous evidence of environmental conditions in taphofacies analyses of fossil assemblages, their resolution potential for paleoenvironmental interpretation should be tested independently in taphonomic analyses. Therefore, these two types of taphonomic data (deposit- level properties and taphonomic signatures), potentially reflecting different processes operating at different scales (Davies et al., 1989a, b; Fu¨ rsich, 1995; Behrensmeyer et al., 2000), are analyzed separately in this study of brachiopod assemblages. Two specific 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 environmental gradients; and (2) the effect of extrinsic environmental factors on brachiopod shell alteration. The first aspect provides the basic framework for genetic interpretation 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 assemblage (Meldahl and Flessa, 1990; Kowalewski et al., 1994; Macchioni, 2000; Wani, 2001). This aspect is very important because it enables testing of differences in preservation potential of brachiopod associations among settings and addressing of questions related to its compositional fidelity (i.e., the quantitative faithfulness of the record of population-community-level features to the original biological signal; Behrensmeyer et al., 2001). In addition to addressing the role of extrinsic factors in alteration and preservation potential of brachiopod associations 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 first part of this work, deposit-level properties are analyzed and interpreted in terms of environmental gradients. In the second part, the variation of taphonomic signatures is evaluated with respect to: (1) previously defined brachiopod deposit types, (2) benthic association types, and (3) deposit types with different packing density. Cor- relation of taphofacies types based on taphonomic signatures with the classification 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) Michalı´k, 1982; Fig. 2). Carbonate deposition is character- Geographic location of sections in the Vel'kaÂ Fatra Mts. 1ÐDedosÆova; ized by reduced terrigenous input and low subsidence re- 2ÐMalyÂ Zvolen; 3ÐBorisÆov; 4ÐBystryÂ potok; 5ÐRaÂztoky; 6ÐKrõÂzna; 7ÐBelianska. TAPHONOMY OF UPPER TRIASSIC BRACHIOPOD DEPOSITS 351 gime. At the beginning of the Jurassic, the depositional re- gime in the Fatric Unit was replaced abruptly by terrigenous sedimentation of the Kopienec Formation (Hettangi- an–Early Sinemurian). The scale of observation in this study includes only a small portion of the intraplatform setting of the Fatric Unit, preserved in a 40-km-long tran- sect in the Vel’ka´ Fatra Mountains (central Slovakia). Here, the Fatra Formation is formed by four large-scale, 6–15-m-thick, shallowing-upward sequences bounded by laterally extensive unconformities (Tomas˘ovy´ch, 2004).

Brachiopods and Benthic Associations

Brachiopods are represented most commonly by the die- lasmatid terebratulid Rhaetina gregaria (Suess), which is characterized by middle-sized to moderately large (15–25 mm long in adult stage), ovate-subpentagonal, smooth bi- convex shells (Michalı´k, 1975; Pearson, 1977). The shell structure of relatively thin valves (0.25–0.4 mm) is punctate. The second common species is represented by middle-sized (20 mm in length), ribbed, semipyramidal spiriferinid Zugmayerella uncinata (Schafhaütl), with punctate shell structure and 0.3–0.5 mm thick valves. Both taxa have cyrtomatodont (interlocking) hinges, which indicates lower susceptibility to disarticulation. However, Zugmay- erella is characterized by the presence of a strophic hinge line and relatively poor development of interlocking char- acters (socket ridges, teeth morphology; Pearson, 1977; Siblı´k, 1998). The shell structure of both species is characterized by the presence of organic-rich primary and secondary layers only; a tertiary layer is not developed. The punctate Rhaetina pyriformis (Suess), impunctate rhynchonellid Austrirhynchia cornigera (Schafhaütl), and in- articulate Discinisca suessi (Gu¨ mbel) are much less common. Four brachiopod associations have been recognized primarily by the taxonomic composition and relative abundance of taxa (Tomas˘ovyćh, 2002). Brachiopods occur either in (1) level-bottom benthic associations, predominantly with soft substrata and dispersed hard (shelly) substrata, including (1a) Rhaetina and (1b) Rhaetina-Zugmayerella associations; (2) benthic associations on the transition between level-bottom and patch-reef structures with higher proportions of hard microsubstrata and high small-scale FIGURE 2ÐBasic lithostratigraphic scheme of the Uppermost Trias- sic±Lowermost Jurassic strata in the Fatric Unit, Central West Car- spatial variability (Rhaetina-Retiophyllia-solenopora- pathians. The subdivision of the Fatra Formation corresponds to four ceans association); or (3) as subordinate components in large-scale shallowing-upward sequences. patch-reef/biostrome associations with dominant retio- phyllid corals providing the highest proportion of hard substrata (Retiophyllia association). It is important to note METHODS that this distinction does not incorporate any taphonomic Sedimentologic, taphonomic, and paleobiologic data of data, and should reflect primary ecologic subdivision of deposits with brachiopod remains were described in the non-random recurrent community relicts. Therefore, the field, from 7 sections (Dedos˘ova, Ra´ztoky, Maly´ Zvolen, same type of benthic association can occur in various types Bystry´ potok, Krı´z˘na, Belianska, and Boris˘ov, Figs. 1B–C, of brachiopod deposits, which are defined according to 3). The term shell concentration is used here to denote de- their taphonomic properties. In the following text, the first posits of any geometry containing a relatively dense accu- two types are designated as brachiopod-bivalve associa- mulation of biogenic hardparts larger than 2 mm (Kidwell, tions, and the latter two as coral associations. The term as- 1991a; Fu¨ rsich, 1995). In order to trace the taphonomic semblage refers here to any group of organisms from a pathways of the brachiopods, it is also necessary to com- sample or locality, with no ecologic meaning (Fu¨ rsich, pare samples in which brachiopods are not the dominant 1990). component. Therefore, the general descriptive term bra- 352 TOMASÆ OVYÂ CH

FIGURE 3ÐSections of the Fatra Formation with brachiopod deposits. Arrows point to brachiopod deposits that were studied in detail (Table 1). From TomasÆovyÂch (2004), with permission of Springer-Verlag. TAPHONOMY OF UPPER TRIASSIC BRACHIOPOD DEPOSITS 353 chiopod deposit is used here to refer to any brachiopod- this, the taphonomic analysis of thin sections allows rec- bearing deposit. Brachiopod deposits have been classified ognition of the preservation of signatures that are not vis- into several types on the basis of bed-level properties (sort- ible on the bioclast surfaces, thus distinguishing multiple ing, packing, orientation, geometry, internal structure, taphonomic events (Brachert et al., 1998; Nebelsick and and thickness). Microfacies types of Dunham (1962) and Bassi, 2000) and excluding the collecting bias due to selec- Embry and Klovan (1972) and bed geometry/internal tive hand sampling. structure have been used for this classification of brachio- For the degree of micritization, bioerosion, and encrus- pod deposit types. tation, a taphonomic grade of good (alteration absent), fair The biofabric and geometry of deposits were described (alteration moderately present), or poor (alteration domi- using the scheme of Kidwell et al. (1986) and Kidwell and nant) has been assigned to each of the brachiopod remains Holland (1991). Allochem abundance was estimated using (Fig. 4). Micritization corresponds predominantly to the the comparative method of Baccelle and Bosellini (1965) destructive type, characterized by an altered and irregular and Scha¨fer (1969). In addition to brachiopods, deposits bioeroded brachiopod surface in contact with a micritic contain other common benthic components, such as bi- rim. Bioclasts entirely covered by thick micritic rims were valves, echinoderms, corals, sponges, and calcareous al- scored as poor. Micritic rims developed only on a limited gae. Because these taxa are characterized by differences portion of the surface were scored as fair. A grade of good in shell structure and mineralogy, only the taphonomic corresponded to the absence of destructive micritization. signatures of the most common terebratulid brachiopod Similarly, in the case of bioerosion and encrustation, spec- Rhaetina, which occurs in all types of brachiopod beds, imens that were bioeroded/encrusted around the whole have been compared. Although the resolution of taphofa- surface were designated as poor, only on some parts as cies types based on a single target taxon may be different fair, and unaffected as good. In order to pool samples into from that based on the total assemblage (Kidwell et al. taphofacies types according to their taphonomic signa- 2001), variation in intrinsic factors is minimized, and dif- tures and compare these groupings with the classification ferences between taphofacies types can be related more di- of deposit types, a Q-mode hierarchical cluster analysis us- rectly to extrinsic environmental factors. For some sam- ing the squared Euclidean distance measure and the ples that contain other brachiopod taxa, additional data Ward method as the clustering technique was performed. are included for the spiriferinid Zugmayerella. For all samples, the proportion of disarticulation and type of shell RESULTS infilling were determined in the field and the degree of fragmentation, micritization, bioerosion, and encrustation Brachiopod Deposits were scored using thin sections. Within-sample variation in preservation (pedicle/brachial valve ratio, disarticula- Based on the deposit-level properties (biofabric, geome- tion of Rhaetina versus Zugmayerella) was obtained from try, and internal structure), 6 types of brachiopod deposits hand sampling of specimens. The proportion of disarticu- are defined: (1) bio-floatstones, (2) pavements of bio-pack- lated specimens and presence of sparitic infillings were stones, (3) bio-floatstones with complex internal structure scored qualitatively. If no articulated shells were ob- (CIS), (4) biointra-packstones with CIS, (5) thick biointra- served, disarticulation was scored as high. When there packstones, and (6) biointra-rudstones. These properties was a mixture of articulated shells and disarticulated are summarized in Table 2. valves, disarticulation was designated as medium. In the (1) Bio-floatstones: These deposits contain 10–30-cm- case of dominance by articulated shells, disarticulation thick bioturbated beds with dispersed to loosely packed, was scored as low. Shell sparitic infillings were recorded poorly sorted, randomly oriented bioclasts (Fig. 5B, D). as abundant or rare. Forty-four beds were studied at this Two brachiopod-bivalve associations occur in this type. In scale (Table 1). the first, the brachiopod Rhaetina gregaria is dominant For quantitative study of the degree of fragmentation, (Fig. 5C) and bivalves (Atreta, Chlamys, Rhaetavicula, bioerosion, micritization, and encrustation, 32 thin sec- Plagiostoma) are subordinate. The second consists of a tions of selected beds were investigated. All brachiopod re- moderately diverse association (Fig. 5A) composed pre- mains above 2 mm were scored from thin sections using dominantly of the terebratulid Rhaetina gregaria, the spi- 50x magnification under a light binocular microscope. Due riferinid Zugmayerella uncinata, and the bivalve Atreta in- to differences in packing density and the limited size of tusstriata. Other brachiopods (Discinisca, Austrirhynchia) thin sections, the number of brachiopod remains ranges and bivalves (Actinostreon, Modiolus, Pteria, Antiquilima, from a low of 10 to a high of 87 specimens. These relatively Rhaetavicula, Plagiostoma, Liostrea) are less abundant. In small sample sizes may cause the alteration frequency dis- both types, brachiopods constitute about 15–25% of rock tributions to be unstable (see Kidwell et al., 2001). Consis- volume. Locally, Rhaetina forms small-scale clusters (10– tent scoring of the taphonomic variables bioerosion and 20 cm in length), 1–2 shells thick, with pedicle openings encrustation in compact and lithified carbonate deposits oriented predominantly downwards (Fig. 5D). In coral as- often is difficult because the outermost shell layers fre- sociations, the significant component is formed by dis- quently fall away from specimens, and the outer and inner persed coral colonies of Retiophyllia and Astraeomorpha, surfaces are often covered by an early-diagenetic micros- calcareous sponges, and solenoporacean algae; brachio- paritic calcite layer. Due to intrastratal pressure solution, pods (Rhaetina) are less common (5% of rock volume). Dis- stylolites also often dissolve the surface of brachiopod persed coral colonies are usually preserved in life orienta- shells. This signature of late-diagenetic origin is easily rec- tion. Disarticulated crinoid ossicles, echinoid spines, gas- ognized in thin sections, can be confused with corrosion or tropods, and ostracodes are common. This type typically bioerosion in specimens sampled by hand. In addition to alternates with the second deposit type. 354 TOMASÆ OVYÂ CH

TABLE 1ÐDatabase of brachiopod deposits of the Fatra Formation (Vel'kaÂ Fatra Mts.), with their characteristic features (rand. ϭ random; mod. ϭ moderate).

Internal Disarticu- Sparitic Sample Thickness Packing Sorting Orientation structure Base Facies lation inﬁlling

DD4 (1) 12 cm dispersed poor random simple wackestone medium rare BP9.4.1a 17 cm loose poor random simple floatstone medium rare BP9.4.2 14 cm loose poor random simple floatstone medium rare BP9.4.3a 7cm loose poor random simple floatstone medium rare BP9.4.4 8cm dispersed poor random simple floatstone low rare BP15.2 25 cm loose poor random simple floatstone medium rare DD5.7 (1) 12 cm loose poor random simple floatstone medium DD16 25 cm dispersed poor random simple floatstone medium B50 25 cm loose poor random simple floatstone low rare S81.2 30 cm loose poor random simple floatstone medium rare R35 10 cm loose/dense poor random simple floatstone medium rare BP9.4.1b 1cm dense moderate concordant simple packstone high rare BP9.4.3b 1cm dense moderate concordant simple packstone high rare BP9.5.1b 1cm loose/dense poor/mod. concordant simple packstone high rare DD4.2 (1) 15 cm loose/dense poor rand./nesting complex erosional floatstone low abundant DD4.6a (2) 15 cm loose poor random complex erosional floatstone low abundant DD5.2 (1) 18 cm loose/dense moderate rand./nesting complex floatstone low abundant DD4 (3) 25 cm loose/dense poor rand./nesting complex floatstone medium DD4.2 (3) 20 cm loose poor rand./nesting complex floatstone medium B28.4 10 cm loose/dense poor random complex erosional floatstone low abundant BP9.5.1 5cm loose/dense poor rand./nesting simple floatstone low BP9.5 2 5cm loose/dense poor rand./nesting simple floatstone low DD4.3 (3) 15 cm dense moderate concordant complex erosional packstone high DD4.4 (3) 20 cm dense good concordant complex erosional packstone high DD4.6b (2) 13 cm dense good concordant complex erosional packstone high DD5 (1) 25 cm dense moderate concordant complex erosional packstone medium DD5.3 (1) 30 cm loose/dense moderate random complex packstone medium abundant S88.5 30 cm loose/dense moderate concordant complex packstone high S93.3 4cm loose moderate concordant simple wackestone high R35.2 20 cm dense moderate random complex packstone medium DD7 200 cm dense bimodal random simple erosional packstone high BP6.3 (1) 30 cm loose/dense bimodal random simple erosional packstone high abundant BP7 42 cm loose/dense bimodal random simple erosional packstone high BP8 28 cm dense bimodal random simple erosional packstone high R10 200 cm loose/dense bimodal random simple erosional packstone high R13 52 cm dense bimodal random simple grainstone high DD13.5 40 cm dense good random simple packstone high K5 30 cm dense good random simple grainstone high DD14.5 25 cm dense good concordant simple erosional rudstone high DD15 20 cm dense good random simple erosional rudstone high BP6.3 (2) 30 cm dense good random simple erosional rudstone high BP14.2 10 cm dense good concordant simple erosional rudstone high S94 28 cm dense good concordant simple erosional rudstone high

(2) Bio-packstone (Pavements): Pavements less than 1 ed brachiopods, several dm in length, are typically present cm thick on the bedding planes of limestone beds are char- in this type. Locally, nesting and stacks of convex-down acterized by loose/dense packing, poor/moderate sorting, disarticulated valves are observed. The typical feature is a and predominantly concordant orientation of bioclasts complex internal structure (CIS) within a single limestone (Fig. 6A–F). Microstylolites and residual-clay seams are bed, where this bio-floatstone alternates with thin (2–5 common; thin Fe-crusts are preserved locally. Bedding mm), well-sorted, allochthonous calcarenitic packstones planes are locally completely covered by small clusters of with erosional bases (Fig. 7A, D). The matrix consists ei- cementing bivalves (Atreta, Fig. 6C) or well-preserved cri- ther of calcilutite (micrite) or well-sorted calcisiltite (un- noidal ossicles (Fig. 6D). Mixed brachiopod-bivalve associ- identifiable bioclastic debris). The brachiopod Rhaetina ations are of moderate diversity and consist of the brachio- gregaria forms almost monospecific concentrations (15– pods Rhaetina gregaria, Zugmayerella uncinata, and Aus- 35% of rock volume, Fig. 7E). Other components (Atreta in- trirhynchia cornigera, and the bivalves Atreta intusstriata, tusstriata, echinoderms, algae) are less common. This de- Rhaetavicula contorta, Palaeocardita austriaca, and Chla- posit type is rather discontinuous laterally and often pass- mys valoniensis. es into or alternates with the biointra-packstones with (3) Bio-floatstones with Complex Internal Structure:5– CIS. 25-cm-thick beds or lenses of this type commonly contain (4) Biointra-packstones with Complex Internal Struc- densely/loosely packed and poorly sorted brachiopods (Fig. ture: 4–30-cm-thick beds of this type commonly display a 7B, C). Three-dimensional clusters with randomly orient- biofabric ranging from moderately/bimodally sorted and TAPHONOMY OF UPPER TRIASSIC BRACHIOPOD DEPOSITS 355

FIGURE 4ÐTaphonomic grades of brachiopods in thin-sections; scale bars ϭ 0.5 mm. (A) Good, without any taphonomic alteration. (B) Bioerosion: poor (intense penetration). (C) Micritization: poor (valve covered by thick micritic rims). (D) Encrustation: fair (some portion is encrusted by foraminifers). (E, F) Encrustation: poor (cementing bivalves and foraminifers). (G) Bioerosion and micritization: fair. (H) Micriti- zation: poor (valve completely covered by destructive micritic rims). (I) Poorly preserved, rounded fragment with high degree of micritization and bioerosion.

TABLE 2ÐDeposit-level taphonomic properties and signatures of 6 basic deposit types and their basic interpretation (concord. ϭ concordant; hab. ϭ habitat; NSWB ϭ normal storm wave base; FWWB ϭ fair-weather wave base).

Taphonomic Bio- Bio-ﬂoatstones Biointra-packstones Thick-bedded signatures and Bio- packstone- with complex with complex biointra- Biointra- bed-level properties ﬂoatstones pavements internal structure internal structure packstones rudstones

Disarticulation medium high low medium-high high high Fragmentation 53.3–100% 81.8% 10–80.4% 85.5–100% 74.7–100% 83.9–100% Bioerosion 11.8–93.4% 2.6% 0–25% 15.9–100% 78.9–100% 64–97.2% Micritisation 0–73.3% 18% 0–22.5% 18.3–58.3% 65.4–93.3% 16.4–94.2% Encrustation 0–16.6% 7.7% 0–12.2% 4.9–18.4% 0–12.2% 1.6–23.1% Sparitic infillings rare rare abundant abundant abundant abundant Assemblage pauci/poly- pauci/poly- paucispecific polyspecific polyspecific polyspecific specific specific Packing dispersed- loose-dense loose-dense dense loose-dense dense loose Sorting poor poor- poor-moderate moderate-good bimodal good moderate Orientation random concordant random random-concord. random concordant Geometry bed pavement bed-lense bed bed bed Internal structure simple simple complex complex simple simple Interpretation within- within- within-hab./census within/out of hab. multiple-hab. multiple/out of habitat habitat hab. Depositional process minor re- winnowing/ storm-reworking- storm-reworking- storm-reworking- long-term working/bio- starvation storm wave storm wave/flow amalgamation wave/ turbation current Setting below below above NSWB above NSWB above NSWB above NSWB NSWB FWWB 356 TOMASÆ OVYÂ CH

FIGURE 5ÐBio-¯oatstone deposits. (A) Upper surface view of bio-¯oatstone with Rhaetina gregaria and Zugmayerella uncinata (BystryÂ potok III-9.4.2); lighting from lower left. (B) Side-view of polished section of bio-¯oatstone with Rhaetina gregaria (BystryÂ potok II-9.4). (C) Outcrop appearance of brachiopod bio-¯oatstone (bed 9.4) overlain by bio-¯oatstone with CIS (beds 9.5±10) (BystryÂ potok II). Scale bar ϭ 1 cm. (D) Upper surface view of nest with Rhaetina gregaria shells (MalyÂ Zvolen 81.2); lighting from lower left. Scale bar ϭ 1 cm.

loosely packed type without preferred orientation (concor- ported fabric, bimodal sorting of allochems (well-sorted dant, oblique, or nested) to well-sorted and densely packed calcarenitic bioclastic-intraclastic debris and poorly sorted type with predominantly concordant orientation of bio- ruditic bioclasts of variable alteration), and random ori- clasts (Fig. 8A, B). The biofabric changes rapidly both hor- entation (Fig. 9A, B). Locally, internal bedding planes are izontally and vertically. The micritic matrix contains distinguishable on the basis of shaly intercalations. This loosely/densely packed calcarenitic allochems (shell de- deposit passes laterally into the 4th or 6th deposit types. bris, intraclasts, peloids, and ooids). Beds are character- The micritic matrix is absent locally. Well-sorted, angular, ized by sharp, irregular, erosional bases, and ﬁning-up- and oval peloids are common; ooids also may be present. ward grading (Fig. 8C). A complex internal structure is Brachiopods represented by Rhaetina gregaria (5–15% of represented by alternation of several cm-thick packstone rock volume) are not the main component in the diverse interbeds with basal erosional surfaces and different lev- spectrum of benthic taxa. Recrystallized bivalve frag- els of sorting and packing. Benthic associations, which are ments are dominant; gastropods, coral fragments or whole represented mostly by a patch-reef Rhaetina-Retiophyllia- coral colonies, red algae, echinoderm ossicles, and fora- solenoporaceans association, and less commonly, by level- minifers are common. bottom (Rhaetina ass.) types, consist of brachiopods (6) Biointra-rudstones: 10–30-cm-thick, very well-sorted (abundant Rhaetina gregaria, less common Zugmayerella beds are characterized by densely packed bioclasts that uncinata), bivalves (Atreta intusstriata, oysters, pectinids), are mainly concordantly and convex-up oriented (Fig. 9C, gastropods, echinoderms, calcareous sponges, corals (Re- D). The lower boundary is sharp (although overprinted by tiophyllia, Astraeomorpha), solenoporacean algae, and cy- stylolites), and signs of grading of size and density of allo- anobacteria (Cayeuxia). chems are locally present. Intraclasts, ooids, and peloids (5) Thick-bedded Biointra-packstones: This deposit type are present in variable amounts. Brachiopods (7.5–25% of is characterized by thick bedding (30–200 cm), clast-sup- rock volume) are represented by Rhaetina gregaria only. TAPHONOMY OF UPPER TRIASSIC BRACHIOPOD DEPOSITS 357

FIGURE 6ÐBio-packstoneÐpavements; scale bars ϭ 1 cm. (A) Upper surface view of bio-packstone pavement with disarticulated Zugmay- erella valves and bivalve Atreta (BystryÂ potok I-12.3); lighting from bottom. (B) Upper surface view of pavement with disarticulated brachiopods (BystryÂ potok III-9.4.1); lighting from bottom. (C) Atreta intusstriata, a cementing bivalve typical of this deposit type. (D) Upper surface view of crinoidal pavement (BystryÂ potok III-9.5.1). (E) Upper surface view of pavement with disarticulated Zugmayerella valves (BystryÂ potok III-9.4.1); lighting from left. (F) Upper surface view of pavement with disarticulated brachiopods (BystryÂ potok III-9.4.1); lighting from right.

Taphonomic Signatures lochems (below 30%, 30–60%, and above 60%, Fig. 11C). In the latter three histograms, the taphonomic grades of fair Quantitative data on the degree of micritization, bioe- and poor were pooled together. The level of significance rosion, fragmentation, and encrustation are presented was estimated using a 95% confidence interval. The dis- and compared using alteration frequency histograms of tinction of sample groupings (taphofacies types) based on individual samples (Fig. 10), deposit types (Fig. 11A), ben- taphonomic signatures and their relationship to the clas- thic-association types (coral and brachiopod-bivalve asso- sification of deposit types based on deposit-level properties ciations, Fig. 11B), and with respect to the proportion of al- were explored using a cluster analysis (Fig. 12). Qualita- 358 TOMASÆ OVYÂ CH

FIGURE 7ÐBio-¯oatstones with CIS; scale bars ϭ 1 cm. (A) Side view of polished section with complex internal strati®cation (CIS) formed by interlayering of bio-¯oatstones with articulated shells with geopetal ®lls and thin well-sorted biointra-packstones with erosional bases (arrows point to erosional boundaries, DedosÆova 4.2). (B) Side view of Bio-¯oatstone with calcisiltic matrix and ¯oating shells with geopetal in®llings (BystryÂ potok II-9.5). (C) Side view of thin-section with bio-¯oatstone with articulated Rhaetina gregaria shells (DedosÆova 4.6a). (D) Side view of polished section with complex internal strati®cation formed by alternation of well-sorted calcarenitic interbeds (arrows) with parautochthonous Rhaetina coquinas (Belianska 26.4). (E) Side views of thin-sections with dominant articulated shells (DedosÆova 4.2 and 4.6).

tive data on disarticulation and shell infilling are present- tions are characterized by higher allochem content (35– ed in Table 1. The distinction of brachiopod-bivalve and 62.5%). coral association is applied only to supposedly autochtho- Disarticulation: The proportion of disarticulation is low nous and parautochthonous deposit types (Bio-float- to medium in Bio-floatstones (Table 1). In this deposit, the stones, Pavements, Bio-floatstones with CIS, Biointra- proportion of articulated nonstrophic shells of Rhaetina is packstones with CIS; see discussion). There is a close cor- higher (50%) in comparison to that of specimens of Zug- respondence between the type of benthic association and mayerella (10–30%). A low proportion of disarticulated its allochem content. Most of the brachiopod-bivalve asso- shells is typical of Bio-floatstones with CIS. Disarticulated ciations fall into the category with low allochem content valves are more common in Pavements (90–95 % in Zug- (below 30%), whereas deposits containing coral associa- mayerella, 50–75% in Rhaetina counted from hand-sam- TAPHONOMY OF UPPER TRIASSIC BRACHIOPOD DEPOSITS 359

FIGURE 8ÐBiointra-packstones with CIS; scale bars: 1 portion ϭ 1 cm. (A, B) Side views of thin-section and polished section, respectively (DedosÆova 4.6b). (C) Side view of polished section with erosional base and grading (DedosÆova 4.4). (D) Side view of thin section, with higher proportion of articulated shells, nesting, and concave-up orientations (DedosÆova 5.2b). (E) Plane view of thin-section (DedosÆova dolina 5). pled specimens), and Biointra-packstones with CIS. Bio-floatstones with CIS, the degree of bioerosion is very Thick-bedded biointra-packstones and Biointra-rudstones low (0–25% per sample, Fig. 11A). Variation in the degree show a scarcity or total absence of articulated shells. of bioerosion is very high in Biointra-packstones with CIS Shell Infillings: The type of shell infilling varies sub- (15.9–100% per sample). The degree of bioerosion is also stantially among individual deposit types. Obviously, only high in Thick-bedded biointra-packstones (78.9–100% per deposits with preserved articulated shells were compared. sample) and Biointra-rudstones (64–97.2% per sample). In Bio-floatstones and Pavements contain articulated shells brachiopod-bivalve associations, 17.4–43.8% of specimens predominantly with homogenous micritic infilling. Bio- are affected by bioerosion (Fig. 11B). In coral associations, floatstones and Biointra-packstones with CIS are charac- taphonomic alteration is higher, but more variable, reach- terized by dominance of complete or partial sparitic infilling up to 93.4% of bioeroded specimens. Although boring ings (Table 1). The micritic parts of shell infilling are mi- frequency in other taxa has not been quantified, bivalve critic or peloidal. Although consistent data on the preser- macroborings (Lithophaga sp.) are very abundant in retio- vation of delicate internal structures (loops) are not phyllid corals, Astraeomorpha, and solenoporacean algae available from all deposit types, it is interesting to note in coral associations. The difference between the degree of that in Bio-floatstones with CIS, loops are preserved in bioerosion of brachiopod-bivalve and coral associations is most of the shells. significant in terms of non-overlapping 95% confidence in- Bioerosion: Variation in the degree of bioerosion in Bio- tervals. Samples with low packing density (below 30%) are floatstones is very high (Figs. 10, 11A). In Pavements and characterized mostly by lower degrees of bioerosion (0– 360 TOMASÆ OVYÂ CH

FIGURE 9ÐThick-bedded biointra-packstones; scale bars: 1 portion ϭ 1 cm. (A, B). Side views of thin-sections with bimodal sorting and high preservational variation (BystryÂ potok 8; 6.3); Biointra-rudstones. (C, D) Side views of thin-sections with concordantly oriented and highly degraded bioclasts (BystryÂ potok 14.2, DedosÆova 14.5).

43.8%) in comparison to samples with higher packing den- minifers (Baccinella irregularis, Lithocodium aggrega- sity (Fig. 11C). tum) and serpulids. In general, the degree of encrustation Micritization: The degree of micritization is typically on brachiopods is very low and relatively uniform in all de- uniform and very low in the Bio-floatstone deposits with posit types (below 25% per sample, Fig. 10). Encrustation CIS (0–22.5%, Fig. 10). Thick-bedded biointra-packstone is mostly absent or very scarce in Bio-floatstone deposits deposits are mostly highly affected by micritization (65.4– with CIS (0–10%). In the coral associations, the degree of 93.3%). The degree of micritization in Biointra-rudstones encrustation (5–17% per sample) is higher in comparison is more variable (16.4–94.2%). The degree of micritization to those in brachiopod-bivalve associations (Fig. 11B). The in Bio-floatstones with CIS is significantly different than highest degree of encrustation is reached in deposits with in Biointra-packstones with CIS, Thick Biointra-pack- higher packing density (23% per sample), although there stones, and Rudstones (Fig. 11A). There is a significant is no significant difference in confidence intervals (Fig. difference between brachiopod-bivalve and coral associa- 11C). tions (Fig. 11B). Micritization in the latter type is more Fragmentation: Levels of fragmentation are mostly high variable and mostly higher (18.3–77.3% per sample), in across all samples and deposit types (Figs. 10, 11). The contrast to the former (0–22.5% per sample). The degree of proportion of fragmentation in Bio-floatstone deposits is micritization is significantly lower in deposits with lower variable, ranging from 53.3–82.6 % in brachiopod-bivalve packing density (Fig. 11C). and 76.5–100% in coral associations. The difference be- Encrustation: Encrustation is mostly very low on bra- tween means of these associations is significant at pϭ95%. chiopods and is characterized by cementing bivalves (Atre- The variation also is high in Bio-floatstone with CIS (10– ta intusstriata) or sessile foraminifers (Tolypammina, 80% per sample). The confidence intervals of mean value Planinvoluta). Although not quantified, qualitatively it of this deposit type do not overlap with those of Biointra- seems that more robust taxa, such as corals or red algae, packstones with CIS, Thick-bedded biointra-packstones, are more heavily affected by encrusting and boring fora- and Biointra-rudstones. These latter types are character- TAPHONOMY OF UPPER TRIASSIC BRACHIOPOD DEPOSITS 361

FIGURE 11ÐAlteration frequency histograms of mean values of taphonomic signatures. (A) Deposit types; for pavements, only data from one sample were available. (B) Benthic associations. (C) Allochem content (packing density).

ized by uniformly high proportions of fragmentation (74.7–100%). The levels of fragmentation in deposits with high packing density (above 30% and 60% of allochems) are higher and more uniform (66.8–100%) in comparison to deposits with lower packing density (10–82.6%). Taphofacies Types Based on Taphonomic Signatures:In order to compare the classiﬁcation of brachiopod-deposit types based on deposit-level properties with different taphofacies types based on taphonomic signatures, cluster FIGURE 10ÐLevels of taphonomic signatures (bioerosion, micritiza- analysis was used for basic distinction of taphofacies types tion, fragmentation, encrustation) in individual samples. The assign- (Fig. 12A). Three clusters with substantial differences in ment of samples to particular deposit types is given at the bottom of mean values of particular taphonomic signatures were ®gure (P. ϭ Pavements). recognized (Fig. 12B). When clusters 1 and 3 were subdi- vided further into subclusters, differences in the mean values were very small (not shown). Therefore, three tapho- 362 TOMASÆ OVYÂ CH

As can be seen from the taphofacies composition (Fig. 12), samples within particular taphofacies types correspond to several deposit types, and particular deposit types can be present in two taphofacies types. However, thick biointra-packstones and biointra-rudstones are consistently characterized by high alteration (Taphofacies 3). Bio-ﬂoatstones with CIS are consistently present in Ta- phofacies 1 and 2 only, characterized by the lowest alteration. Bio-ﬂoatstones and biointra-packstones with CIS are present in Taphofacies 1 and 3. In summary, the cor- respondence between the assignment of particular samples either to the deposit types or to the taphofacies types is relatively poor, although some deposit types are consistently characterized by relatively stable levels of taphonomic alteration.

DISCUSSION Brachiopod-deposit Types In the following text, the genesis of the brachiopod-deposit types is interpreted and classified (genetic terms in parentheses) according to a process-related approach used in classification of shell concentrations (see Speyer and Brett, 1991; Fu¨ rsich and Oschmann, 1993; Fu¨ rsich, 1995). (1) Thin-bedded Bio-floatstones: The fabric of brachiopods, together with the absence of high-energy sedimentary structures, indicates an environment below storm wave base, suggesting that these deposits probably under- went only minor post-mortem reworking. The lithologic homogeneity of this deposit type, co-occurrence of benthic taxa that are characterized by similar ecological require- ments, and the absence of evidence for census, environmental, or biostratigraphic condensation (Kidwell, 1998) allow an interpretation as autochthonous within-habitat time-averaged assemblages. The high abundance of brachiopods can be linked to their relatively high productivity and/or the low rate of destruction of their dead shells (primary biogenic deposit). Small-scale brachiopod clusters probably are preserved in original position. Close spatial association with Pavements indicates the proximity of maximum storm wave base. (2) Bio-packstone—Pavements: The restriction of micro- FIGURE 12Ð(A) Cluster analysis of 32 samples with taphonomic signatures (bioerosion, micritization, encrustation, and fragmentation) as bial rims and sessile foraminifers to the upper surfaces of the variables. On the left side, corresponding deposit types based on bioclasts indicates a stable position on the seafloor. This, deposit-level properties are shown. (B) Alteration frequency histo- along with the absence of sorting and rounding, suggests grams of 3 clusters recognized by the cluster analysis. that disturbance of the sea floor was minor. The origin of this biofabric may be due to episodic distal storm flow winnowing (winnowed deposit) and/or periodically reduced facies types based on taphonomic signatures can be distin- background rates of sedimentation (sediment-starved de- guished: (1) Taphofacies 1, characterized by moderate deposit). Microstylolites indicate a significant diagenetic gree of bioerosion/micritization (Cluster 1); (2) overprint, leading to the enrichment of fossil hardparts. Taphofacies 2, with the lowest degree of bioerosion/micri- (3) Bio-floatstones with Complex Internal Structure: The tization (Cluster 2); and (3) Taphofacies 3, characterized complex internal structure indicates episodic high-energy by the highest degree of bioerosion/micritization (Cluster events, most probably of storm origin. Nesting in various 3). The rank order of individual categories in histograms is directions and convex-down orientation of valves indicate mostly similar. Fragmentation is the highest, bioerosion the presence of turbulent/vortical flow and suspension set- and micritization follow, and the proportion of encrusta- tling (Middleton 1967; Futterer, 1982). Because brachio- tion is the lowest. The proportion of bioerosion and micri- pod shells are oriented inconsistently with respect to the tization is substantially lower in the first two taphofacies antero-posterior axis, it is improbable that they are con- in comparison with Taphofacies 3. In addition, the propor- served in their life orientation. The features typical of this tion of bioerosion and micritization is substantially higher deposit type (laterally restricted monospecific assemblage in Taphofacies 1 in comparison with Taphofacies 2. of concentrated, predominantly articulated shells, closely TAPHONOMY OF UPPER TRIASSIC BRACHIOPOD DEPOSITS 363 associated with erosional surfaces) are comparable with a in some cases, indicate higher current activity. The bra- catastrophic mortality model proposed by Tsujita (1995) chiopods and other bioclasts are probably mostly alloch- for the origin of bivalve shell clusters in the Late Creta- thonous or can represent residual in-situ components ceous Bearpaw Formation. The main mechanism is relat- when the local setting changed from low- to high-energy ed to a combination of a clustered or gregarious life habit conditions. Fossil assemblages are strongly biased toward of shell producers and storm-induced sea-floor scouring the more resistant shelly remains. This long-term current/ and subsequent filling, preferentially localized around to- wave deposit was generated and accentuated by long- pographic irregularities produced by clusters of live shells term, fair-weather high-energy and episodic storm-flow (Johnson, 1957; Aigner and Futterer, 1978; Goldring and processes, as is indicated by its spatial association with Aigner, 1982). Therefore, simple-event mechanical re- peritidal and shoal facies (skeletal sand banks). working and rapid in-situ burial of living brachiopod clus- Processes Leading to the Origin of Brachiopod Deposits: ters in a scour-generated depression could be responsible Based on the interpretation of taphonomic properties of for the origin of this deposit type in environments with deposits, a process-related concept of classification of ta- low-energy background conditions. Therefore, time-aver- phofacies or shell concentrations of epicontinental seas is aging is minimal and the deposits represent census as- appropriate (Speyer and Brett, 1991; Fu¨ rsich and Osch- semblages produced by catastrophic mortality (e.g., Brett, mann, 1993; Fu¨ rsich, 1995). Thus, fossil assemblages with 1990). Waves generated by storms may be the best expla- the terebratulid brachiopod Rhaetina gregaria are classi- nation (storm wave deposit). fied into six basic deposit types: (1) autochthonous prima- (4) Biointra-packstones with Complex Internal Struc- ry biogenic, (2) autochthonous winnowed or sediment/ ture: As in the previous deposit, complex internal struc- starved, (3) parautochthonous storm wave, (4) parauto- ture with internal erosional boundaries suggests episodic chthonous storm wave/storm flow, (5) amalgamated storm storm activity. Moderate or bimodal sorting, loose pack- reworked, and (6) allochthonous long-term current/wave ing, absence of preferred orientations or nesting, and con- type. Described types of brachiopod deposits can be ar- vex-down orientations indicate more storm wave influ- ranged on a short-term scale according to the degree of ence, whereas well-sorted and densely packed bioclasts mechanical reworking and transport of bioclasts (Fig. 13) with predominantly concordant and convex-up orienta- as simple event deposits in a simplified chart, with the ex- tions are more indicative of storm flow activity. In the lat- ception of amalgamated storm-reworked deposit. In addi- ter case, some exotic allochems (ooids, oncoids, or intra- tion to this aspect, brachiopod deposits differ in relative clasts) indicate short-term transport in storm-induced degrees of time-averaging, ranging from census and with- currents and deposition of suspended material in graded in-habitat time-averaged to environmentally condensed bedding (Aigner, 1979). In addition to different intensity of types (Kidwell and Bosence, 1991). The differences in ex- storm reworking, the original difference in the species trinsic factors reflect mainly variations in the degree of re- composition (i.e., higher proportion of corals, sponges, and working and net rate of sedimentation. algae) probably also affected the difference in biofabric be- Three groups of brachiopod deposits, corresponding to tween Bio-floatstones with CIS and Biointra-packstones three main bathymetric settings with respect to position of with CIS. High spatial heterogeneity in the species distri- fair-weather, storm, and maximum storm wave bases, can bution across short distances indicates that the transport be distinguished according to the degree of reworking, in- of benthic remains was not significant. Rapid lateral ternal microstratigraphy and the original composition of changes in biofabric are attributable to the irregular to- the benthic associations occupying the sea-floor. pography of patch-reefs and faunal patchiness, leading to (1) Relatively undisturbed deposits originating under localized storm-induced scour-and-fill mechanisms. This long-term low-energy conditions (primary biogenic and storm wave/flow deposit thus probably contains both par- winnowed or sediment-starved types) occur in environ- autochthonous and allochthonous bioclasts (mixed assem- ments between maximum and average storm wave base, blage). with prevailing background taphonomic processes (biotur- (5) Thick-bedded Biointra-packstones: Paleoecologic and bation, background burial rate, net rate of sedimentation). sedimentologic data suggest a composite origin of multi- This setting was inhabited mainly by moderately diverse ple-habitat, environmentally condensed assemblages. brachiopod-bivalve associations (level-bottom Rhaetina- This represents an amalgamated storm-reworked deposit Zugmayerella and Rhaetina associations). Coral associa- with the compounded effects of background and episodic tions on the transition to patch reefs with dispersed coral processes. Locally, large overturned and complete branch- colonies and algae rarely occurred in this zone. Due to poor ing coral colonies indicate the absence of prolonged trans- storm influence, brachiopods did not experience complex port. The microintraclastic, well-sorted matrix, and local burial/exhumation histories. The distinction between absence of micritic mud points to long-term effects of a rel- loosely (primary biogenic) and densely packed (winnowed atively higher-energy setting close to normal wave base or or sediment starved) deposit types can be explained by frequent multiple storm-reworking. The presence of com- changes in the net rate of sedimentation, leading to a dif- plete and overturned branching coral colonies indicate pe- ferent degree of time-averaging of two deposit types. Sim- riods with lower background energy and episodic storm ilar control on the preservation of autochthonous terebra- burial. However, amalgamation and homogenization of tulid shell beds is known from the Upper Muschelkalk of deposits led mostly to the destruction of original features Germany (Aigner et al., 1978; Hagdorn and Mundlos, such as bedding or superposition of individual events. 1982), indicating that the changes in the net rate of sedi- (6) Biointra-rudstones: Good sorting, absence of fine ma- mentation are the dominant taphonomic factor shaping trix, dense packing, and rounding indicate long-term high- biofabric of autochthonous brachiopod deposits. energy influence. Predominantly convex-up orientations, (2) Storm-reworked deposits, with conflicting effects of 364 TOMASÆ OVYÂ CH

FIGURE 13ÐAn idealized sequence of taphonomic events during the formation of brachiopod deposits, from autochthonous origin to allochthonous reworking. TAPHONOMY OF UPPER TRIASSIC BRACHIOPOD DEPOSITS 365 background and episodic taphonomic processes, are rep- mentation also has been documented in Middle Triassic resented by storm-wave and storm-flow types with CIS, shell beds (Aigner et al., 1978; To¨rok, 1993). In contrast, deposited below fair-weather wave base, and amalgamat- the shell fillings in Bio-floatstones are mostly micritic. If ed storm-reworked type, affected by storm reworking of the interpretation of these deposits as that of an autoch- higher intensity (e.g., Aigner, 1982; Norris, 1986; Parsons thonous primary biogenic type holds, shells were not bur- et al., 1988; Johnson, 1989; Drummond and Sheets, 2001) ied rapidly by storm events (i.e., they were exposed for at deposited nearer to the fair-weather wave base. They orig- least some time on the sea-floor). The residence time was inated in settings above the normal storm wave base. sufficient to completely fill the articulated shell with mi- These settings were predominantly inhabited by the coral critic mud. The process of filling of brachiopod shells is associations (Rhaetina-Retiophyllia-solenoporaceans and usually explained simply by infiltration of sediment, draft Retiophyllia associations) or paucispecific brachiopod-bi- filling (draft stream created by external turbulence, Sei- valve associations (Rhaetina associations). They are pre- lacher, 1971), or sediment traps, which are characterized served as parautochthonous storm-reworked deposits. Be- by narrow gaps (e.g., pedicle opening) and overhangs (e.g., cause coral associations were more common in this setting high valve convexity). In the latter case, due to the differ- than brachiopod-bivalve associations, they had a higher ence in density gradient of suspended sediment on the sea probability of reworking. The presence of well-defined, floor and within the shell, there is a higher input of sedi- centimeter-scale interbeds in Bio-floatstones with CIS ment into the sediment trap (shell), which consequently is and Biointra-packstones with CIS indicates that storm ac- overtrapped with fine sediment (Gardner, 1980a, b). The tivity was less intense than in amalgamated Thick-bedded type of shell infilling thus reflects mainly the rate of buri- biointra-packstones. In addition to the differential storm- al/residence time on sea/floor. induced reworking and burial, original differences in com- Bioerosion/micritization: Levels of bioerosion and micri- position of the benthic associations and patchy distribu- tization are closely associated, as evinced by vacant micro- tion of live brachiopod populations led to the differential borings that have been filled with micritic cement; al- biofabric between Bio-floatstones with CIS and Biointra- though other biologically mediated precipitation processes packstones with CIS. The storm activity probably caused may influence micritization (Kobluk and Risk, 1977). Low burial-induced catastrophic mortality of brachiopod popu- degrees of bioerosion and micritization in brachiopod-bi- lations in this setting. valve associations in Bio-floatstone deposits, interpreted (3) Deposits originating under long-term high-energy to represent an autochthonous deposit unaffected by epi- conditions, in environments above fair-weather wave sodic storm burial, point to some intrinsic or extrinsic fac- base, are represented by long-term current/wave type tor(s) inhibiting microboring organisms from brachiopods with compounded signatures of destructive background exposed on the sea floor. High net rate of sedimentation is and episodic taphonomic processes. Transport out of hab- improbable because of the ecological structure of benthic itat or gradual shallowing might lead to the origin of these associations dominated by epifaunal suspension-feeders deposits, where preservation of brachiopods has been (Tomas˘ovyćh, 2002). Light limitation can be excluded be- overprinted by long-term higher energy conditions. cause of green algae are common. Other ecological factors, such as levels of nutrient supply, may control the distri- Taphonomic Signatures bution of bioeroders (Hallock and Schlager, 1986; Hallock, 1988; Lescinsky et al., 2002). The intrinsic cause can be re- Disarticulation: The highest proportion of articulation is lated to valve thinness and/or relatively high rate of de- present in deposit types that are interpreted as storm- struction of brachiopod shells when they are exposed on wave deposits associated with rapid burial. The medium the sea floor. It is known that boring organisms exhibit proportion of disarticulation in Bio-floatstone deposits, in- highly selective settlement behavior for thicker and more terpreted as an autochthonous primary biogenic deposit, durable shells (Best and Kidwell, 2000b). High rates of points to higher activity of disturbing biota and/or higher shell destruction due to organic matrix breakdown (shell intrinsic rate of destruction. In autochthonous pavements, maceration), which occur rapidly in modern temperate the higher proportion of disarticulation indicates relative- settings, could lead to the effect that when exposed on the ly longer residence times on the sea-floor compared to sea-floor, brachiopod shells decomposed rapidly and thus thin-bedded bio-floatstones. The difference in the propor- provided poor substrates for boring organisms (Daley, tion of disarticulation between Rhaetina and Zugmayerel- 1993). la reflects the differences in hinge strength. In deposits af- The association of low degrees of bioerosion with depos- fected by transport, complex burial/exhumation history, its with low allochem content (i.e., high proportion of mi- and long-term high-energy influence, the proportion of critic mud) agrees with results of other studies (Vogel et disarticulation is high, thus, the proportion of disarticula- al., 1987; Best and Kidwell, 2000a, b). In soft-bottom en- tion can be positively correlated with residence time on the vironments, the scarcity of microborings can be explained sea floor, which is directly related to the reduced rate of by unstable conditions at the sediment-water interface, sedimentation and higher intensity of episodic or long- due to higher turbidity and mud re-suspension. Therefore, term high-energy events. environmental factors related to fine-grained sediment ac- Shell Infillings: A high proportion of articulated shells, cumulation can have both ecological and taphonomic ef- completely or partially filled with sparitic calcite, is typical fects on observed levels of bioerosion. On one hand, high of the Bio-floatstones with CIS. The interpreted storm- turbidity and unstable substrate can primarily decrease wave origin associated with rapid burial suggests that the ecological abundance of borers, and on the other hand, penetration of the mud was inhibited. The association of fine-grained sediment has a relatively high potential to sparite-filled brachiopod shells with higher rates of sedi- bury shells and protect them from taphonomic destruction 366 TOMASÆ OVYÂ CH related to bioerosion. There is probably a complex rela- Biointra-rudstones are consistently characterized by high tionship between several inter-related factors controlling proportion of bioerosion, micritization, and fragmentation. the activity of boring organisms, so the interpretation of This indicates that deposits originating near fair-weather bioerosion intensity is not always straightforward. wave base and characterized by long-term or repeated In coral associations, the degree of bioerosion of brachio- short-term reworking were affected by the highest bra- pods is relatively high. Here, environmental factors can be chiopod alteration. explained in terms of higher proportions of hard and sta- In regard to the intensity of brachiopod alteration ble substrata and reduced turbidity/sedimentation rates caused by bioerosion/micritization in autochthonous and (Pandolfi and Greenstein, 1997). Scattered brachiopod parautochthonous deposit types, two different patterns shells preserved in life positions in inter-corallite voids are can be recognized, related to the difference in composition often micritized and encrusted by foraminifers and serpu- of benthic association types (brachiopod-bivalve versus lids, suggesting different taphonomic pathways in patch- coral). In brachiopod-bivalve associations, bioerosion and reef/biostrome associations in comparison to level-bottom micritization are absent or scarce, whereas coral associa- associations. tions with small-scale patch-reefs/biostromes, bioerosion/ Encrustation: The degree of encrustation is interesting- micritization, and fragmentation are more common (Fig. ly very low. In general, Triassic level-bottom encrusting 11B). Higher alteration of brachiopods in coral patch- associations are not very common, although encrusting as- reefs/biostromes probably is related to several inter-relat- sociations known from Rhaetian Hybe Formation (Hronic ed factors, including the intrinsically high proportion of Unit of the Central West Carpathians) are highly diverse hard substrata and increased effect of taphonomic feed- (Taylor and Michalı´k, 1991). Restricted diversity of en- back (higher shelliness, lower proportion of soft sediment, crusters is probably also the effect of environmental insta- increased bioerosion; see Kidwell, 1991b), bottom-water bility and habitat restriction of depositional setting of the quality suitable for borers and encrusters, and frequent Fatric Unit (Michalı´k, 1982, Tomas˘ovyćh, 2002). Another storm-reworking (burial-exhumation), leading to longer explanation can be related to the lowered productivity re- residence time on the sea-floor. Similarly, the difference gime, which seems to be negatively correlated with en- between deposits with low- and high-packing density (Fig. crustation intensity in modern seas (Birkeland, 1977, 11C) can reflect the importance of several inter-related 1989; Lescinsky et al., 2002). The abundance of Retiophyl- factors, such as substrate type, rate of sedimentation, and lia coral patch-reefs should indicate the absence of eutro- turbidity. Therefore, due to between-setting variation in phic conditions. Shell orientation, size, and ornamentation rates of taphonomic destruction, it seems that Upper Tri- are significant factors determining the proportion and assic brachiopods exhibited a difference in preservation composition of encrusting communities (Kesling et al., potential between high-/low-energy and hard-/soft-bottom 1980; Spjeldnaes, 1984; Alvarez and Taylor, 1987; Alex- settings, which means that their compositional fidelity ander and Scharf, 1990; Powers and Ausich, 1990). For ex- can differ substantially. The depth-/substrate-related dis- ample, Bordeaux and Brett (1990) observed that smooth tribution pattern of brachiopod fossil associations is prob- punctate terebratulids were relatively free of encrusters in ably biased due to this extrinsic difference in their preser- contrast to other brachiopods. However, it is also possible vation potential. Such bias potentially can be detected that the encrusting communities were dominated by non- when there are significant between-setting differences in preservable taxa with low preservation potential, leading taphonomic alteration within one taxonomic group char- to the substantial decrease of encrustation in the fossil as- acterized by similar intrinsic preservation properties (i.e., semblages (Lescinsky, 1993). durability). Overall Taphonomic Alteration: The significant differences between the intensity of taphonomic alteration IMPLICATIONS FOR UPPER TRIASSIC among particular samples pooled either according to their BRACHIOPOD DISTRIBUTION PATTERNS deposit-level properties (Fig. 11A) or according to their taphonomic signatures (Fig. 12) indicate inherent varia- Only a few local studies exist on the pattern of brachio- tions in taphonomic pathways related to extrinsic environ- pod distribution from the Triassic–Jurassic boundary mental factors. Variation of taphonomic signatures in (Michalı´k et al., 1991; Sandy, 1995; Dulai, 2001, 2003; To- samples can be very high and does not strictly follow the mas˘ovyćh, 2001). The evaluation of brachiopod distribu- distinction of deposit types based on their deposit-level tion patterns before and after the end-Triassic event and properties (Fig. 12). This is probably due to the fact that the study of their survival and recovery inevitably depend biofabric primarily reflects final depositional processes on understanding the taphonomic processes controlling (the rate and permanence of burial), whereas some tapho- their preservation in fossil assemblages. It has been sug- nomic signatures reflect variation in the nature of pre- gested that the distribution patterns of Upper Triassic burial environmental conditions (Davies et al., 1989a, brachiopod associations can be correlated with depth-re- Fu¨ rsich, 1995). For example, sorting, packing, and geom- lated environmental gradients (Golebiowski 1991, Sandy, etry of a deposit can be formed during one short-term 1995). According to Sandy (1995), based mainly on bra- high-energy event. In contrast, the degree of bioerosion or chiopod distribution in the uppermost Norian–Rhaetian encrustation will depend on more long-term ecologic/taph- Ko¨ssen Formation of the Eastern Alps, short-looped tere- onomic conditions. The lowest brachiopod alteration, as- bratulids (represented by Rhaetina gregaria) show a sociated with low levels of bioerosion/micritization, en- strong preference for shallower water, transgressive por- crustation, and disarticulation, is associated with the de- tions of the sequence, which represent high-energy envi- posit type that was affected by sudden burial (Bio-float- ronments. Although the Rhaetian Fatra Formation does stones with CIS). Thick-bedded Biointra-packstones and not contain deep subtidal or offshore deposits (character- TAPHONOMY OF UPPER TRIASSIC BRACHIOPOD DEPOSITS 367 ized in the Ko¨ssen Formation by dominance of rhyncho- lower predation susceptibility (Kidwell, 1990; Copper, nellids, athyrids and long-looped terebratulids), it seems 1997). However, the high organic content of brachiopod that the complete bathymetric range of Rhaetina gregaria shells (Curry et al., 1989) may increase the rate of shell is preserved. However, taphonomic analysis suggests that breakdown significantly due to decomposition of organic although a Rhaetina-dominated association is present in matrix (shell maceration) and thus decrease the preser- the uppermost part of the environmental gradient in the vation potential during processes in the taphonomic active West Carpathians, autochthonous/parautochthonous oc- zone (Alexandersson, 1979; Kidwell, 1990; Glover and currences are restricted to settings below the fair-weather Kidwell, 1993; Kidwell and Brenchley, 1994, 1996). Al- wave base with background low-energy conditions, often though actualistic data about the rate of destruction of in association with soft macrosubstrata (Rhaetina and modern brachiopods are derived mostly from temperate Rhaetina-Zugmayerella associations), or in coral associa- siliciclastic settings, studies show brachiopods decompose tions. Fragments or disarticulated valves of Rhaetina also very rapidly due to the breakdown of organic matrix (i.e., are preserved in coarse-grained high-energy deposits orig- on the scale of several months, see Collins, 1986; Gaspard, inating near or above the fair-weather wave base (Thick- 1989, 1996; Emig, 1990; Daley, 1993). However, the dura- bedded biointra-packstones and Biointra-rudstones), but bility of articulate brachiopods in carbonate environments the statement about their original presence in this habitat is poorly known. The data presented here provide some remains inconclusive. On one hand, high-energy, hard- implications about taphonomic pathways and preserva- bottom settings are characterized by a higher rate of de- tion potential of articulate brachiopods in an Upper Tri- struction, which leads to higher alteration of brachiopods assic carbonate setting. (see discussion), even if they are preserved in their origi- Relatively complex alteration patterns of brachiopods in nal habitat. On the other hand, transport out of habitat or Biointra-packstones with CIS, Thick-bedded biointra- environmental condensation (e.g., Jeram model, see Sei- packstones, and Biointra-rudstones indicate that brachio- lacher, 1985) may result in brachiopods being preserved in pods were affected by substantial destruction due to bioe- such high-energy deposits. rosion and micritization (associated with cement precipi- Sedimentologic and deposit-level taphonomic properties tation), suggesting that they could withstand long-term give more support to the latter view. Although an on- exposure to destructive agents. Therefore, this supports shore–offshore distribution pattern of brachiopod associa- the idea that in carbonate settings some other taphonomic tions is not disregarded, the resolution of the habitat pref- pathways (e.g., cement precipitation in inter-fiber voids) erence of Rhaetina gregaria is substantially enhanced in can exclude rapid shell maceration and increase shell du- this study. Nevertheless, when Upper Triassic and Early rability. This is especially supported by the fact that tere- Jurassic brachiopod associations are compared in time-en- bratulids (Rhaetina) and spiriferinids (Zugmayerella) with vironment diagrams (Sandy, 1995), this difference is more very thin and punctate valves should be more susceptible important, because habitats with originally different to rapid shell maceration than brachiopods with impunc- depths can be incorrectly considered as bathymetrically tate valves (see Emig, 1990). Relatively lower rates of de- equivalent. Based on the dominant occurrence of rhyncho- struction are probable also for modern terebratulid bra- nellids in shallow, high-energy habitats in the Early Ju- chiopods from the SW Brazilian tropical carbonate shelf rassic, in contrast to the dominance of short-looped tere- (Kowalewski et al., 2002). These data point to the possibil- bratulids in the Upper Triassic, Sandy (1995) hypothe- ity that inherent resistance to destruction also is environ- sized that rhynchonellids replaced short-looped terebra- ment-dependent and distribution patterns of brachiopods tulids in this habitat after the end-Triassic mass in siliciclastic and carbonate settings can be biased due to extinction. Therefore, confirmation of this habitat replace- different taphonomic pathways (e.g., variations in rate of ment hypothesis must first be tested by assessing the de- organic-matrix breakdown and cement precipitation due gree of taphonomic overprint on distribution of Upper Tri- to differences in pore-water chemistry and concentration assic short-looped terebratulid-dominated associations, and type of organic matrix, see Mitterer and Cunningham, originally interpreted as inhabiting shallow, high-energy 1985). Although this is intuitive (e.g., decrease in the du- settings in the Eastern Alps and other regions. If there are rability of aragonitic shell due to undersaturation of calci- temporal shifts in environmental preferences of particular um carbonate), it is important to note that due to cement taxa (e.g., before and after mass extinction), it is necessary precipitation in inter-crystalline voids, the resistance to to test if occurrences in onshore settings (mostly associat- destruction potentially can be increased during tapho- ed with high-energy/hard-bottom conditions) are substan- nomic processes (Alexandersson, 1972; Cutler and Flessa, tially overprinted by taphonomic processes. 1995). Reduced thickness of brachiopod shell beds is supposed PRESERVATION POTENTIAL OF BRACHIOPODS to represent additional evidence for lower durability/lower degree of time-averaging of brachiopods (Kidwell and In addition to variations in extrinsic environmental fac- Brenchley, 1996). In the framework of long-term trends in tors (e.g., rate of burial, water chemistry, bioturbation, ac- shell-bed thickness, the described brachiopod deposit tivity of boring organisms), another factor influencing the types of the Fatra Formation that can be considered as preservation potential of dead organisms is their inherent shell concentrations (i.e., characterized by dense packing resistance to destruction (Kowalewski, 1996a). The inher- of brachiopods) are typical of archaic shell-bed types with ent potential to preserve articulated brachiopods in the low thickness (mostly up to 50 cm, Table 1). This confirms fossil record is positively influenced by their stable low- the idea that the limited physical scale and low taphonom- magnesium calcitic mineralogy, type of hinge, lack of liga- ic complexity of brachiopod shell beds can be related to ment, macroscopic body size, gregarious settlement, and their organic-rich shell microstructure and life habits (rel- 368 TOMASÆ OVYÂ CH atively low fecundity and limited distribution in high-en- traplatform carbonate settings of the Fatra Formation ergy environments, see Kidwell and Brenchley, 1994, were classified according to the fabric criteria of whole as- 1996; Simo˜es et al., 2000). semblages into 6 deposit types, which are interpreted as: However, some of the brachiopods in this study exhibit- (1) autochthonous (primary biogenic and winnowed or ed substantial shell alteration in some deposit types, in sediment-starved); (2) parautochthonous storm-reworked contrast to typical Paleozoic brachiopod shell beds. In (storm wave, storm wave/flow and amalgamated storm re- most samples, dense packing (i.e., shelliness, Cummins et worked); and (3) allochthonous (long-term current/wave) al., 1986; Davies et al., 1989b) can be explained either by deposits. Their distribution on the bed scale correlates sudden burial by storm waves (leading to catastrophic with a depth-related environmental gradient with respect mortality and rapid escape from the taphonomic active to the positions of fair-weather wave base, average storm zone; Bio-floatstones with CIS) or by secondary enrich- wave base, and maximum storm wave base, and thus pro- ment by storm currents (Pavements and Biointra-pack- vides high-resolution information for paleoenvironmental stones with CIS). Therefore, presented data indicate that reconstruction. Extrinsic variation contributing to the dif- preservation potential/durability of Upper Triassic bra- ferent fabric, geometry, and internal structure is related chiopods in carbonate settings was relatively higher in to: (1) rate of sedimentation; (2) water energy related to re- comparison to that of modern brachiopods from siliciclas- working and burial; and (3) original differences in distri- tic settings, which can exhibit substantial taphonomic al- bution and composition of life associations. Because coral teration and show signs of cement precipitation in borings, associations were more common above normal storm wave although reduced thickness can indicate the presence of base, they had a higher probability of reworking than the some other factors limiting their durability/potential to brachiopod-bivalve associations. Fossil assemblages pre- form thick shell beds. served in brachiopod deposits record a wide range of tem- The inherent durability of brachiopods has important poral resolutions, ranging from census to environmentally implications for the quality of their fossil record. Unal- condensed types. In storm-reworked types, brachiopod tered and relatively uniform preservation of fossil brachio- populations commonly were affected by catastrophic mor- pods typically is explained by a low rate of taphonomic de- tality due to rapid burial and have limited time-averaging. struction and completeness of brachiopod assemblage is (2) Based on taphonomic signatures, three taphofacies supposed to be relatively good. However, if brachiopods types were recognized. This distinction, which does not disintegrate into calcitic fibers faster than they accrue strictly follow the distinction based on the fabric criteria, post-mortem alteration due to bioerosion/micritization (see Kidwell and Brenchley, 1996), then a fossil assem- probably is because the biofabric primarily reflects final blage of well-preserved brachiopods must have been brief- depositional processes (the rate and permanence of buri- ly exposed on the sea-floor, and had a relatively rapid es- al), whereas some taphonomic signatures reflect variation cape from the taphonomic active zone (e.g., burial due to in the nature of pre-burial environmental conditions. The bioturbation). For example, Fu¨ rsich and Pandey (2003) lowest degree of alteration, associated with low levels of described an Upper Jurassic siliciclastic shell bed with bioerosion, micritization, encrustation, and disarticula- well-preserved terebratulid brachiopods, interpreted as tion, is uniformly linked to deposits that were affected by part of a maximum flooding zone deposit, which originat- sudden storm-induced burial. In respect to the type of ben- ed under reduced net rate of sedimentation. Therefore, thic association and substrate type, the lowest alteration they assumed that assemblages are highly time-averaged. is present in deposits containing brachiopod-bivalve asso- This can have important implications for the durability of ciations and deposits with large proportions of micritic brachiopods (i.e., they did not decompose very rapidly in mud. Settings with higher proportions of micritic mud (as- the described setting). However, because taphonomic de- sociated with brachiopod-bivalve associations) are char- struction in terms of disarticulation, fragmentation, or acterized by very low degrees of bioerosion and micritiza- bioerosion is low, the reverse interpretation can be sup- tion, in contrast to hard-bottom settings (associated with posed (i.e., they degraded so rapidly that they could not ac- coral associations). This probably results from mainly ex- crue any substantial bioerosion). This distinction in inter- trinsic factors related to lower turbidity, higher proportion pretation of brachiopod durability is important, since the of hardparts, and higher storm reworking in the latter quality of resulting fossil assemblage will be substantially habitats. This effect can lead to differential between-habi- different (see Kowalewski, 1996b, 1997). In the case with a tat alteration of brachiopods. low rate of shell destruction, the probability of a higher de- (3) Autochthonous/parautochthonous occurrences of gree of time-averaging is greater. In the case with a higher benthic associations dominated by the short-looped tere- rate of shell destruction, the temporal resolution of the bratulid Rhaetina gregaria are typical of settings below brachiopod assemblage probably will be higher (i.e., it will the fair-weather wave base, with background low-energy be characterized by more limited time-averaging and conditions, in contrast to high-energy/hard bottom occur- higher compositional fidelity). For example, in spite of rences of this association type from other regions. The dif- high rate of destruction of punctate brachiopods in the Pa- ference in preservation potential of brachiopod assemblag- cific Northwest (Daley, 1993), their rank abundances and es due to extrinsic factors (e.g., between hard- and soft- size-frequency distributions can be represented faithfully bottom settings) can bias substantially understanding of in the death assemblages (Kowalewski et al., 2003; Toma- their ecology and temporal shifts in their environmental s˘ovyćh, in press, a). preferences. (4) Substantial taphonomic alteration of brachiopods CONCLUSIONS due to bioerosion/micritization in some deposit types pro- (1) Uppermost Triassic fossil assemblages with the ter- vides the evidence that they can resist certain levels of de- ebratulid brachiopod Rhaetina gregaria from shallow in- struction. Actualistic data about very rapid destruction of TAPHONOMY OF UPPER TRIASSIC BRACHIOPOD DEPOSITS 369

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