Cretaceous-Paleocene transition along a rocky carbonate shore: Implications for the Cretaceous-Paleocene boundary event in shallow platform environments and correlation to the deep sea

In shallow-water limestones, stratigraphically fairly complete Cretaceous-Paleocene (K/P) boundary intervals are rarely preserved because erosion and nondeposition resulted in major hiatuses that do not warrant in-depth study of the boundary transition. Consequently, shallow-water habitats are rarely documented and little understood with respect to the aftermath of the K/P boundary event. It is well known that Late Cretaceous biocalcifiers and other shelled biota became extinct (e.g., globotruncanids, rudists, ammonites, belemnites; e.g., Keller et al., 1996; Johnson et al., 2002; Steuber et al., 2002; Keller, 2005), whereas corals, red algae, bryozoans, and skeletal sponges survived without major diversity loss (Håkansson and Thomsen, 1999; Kiessling and Baron-Szabo, 2004; Sogot et al., 2013). Coral–red algal reefs flourished again at least during the middle to late Danian (Vecsei et al., 1998; Perrin, 2002; Baceta et al., 2005; Aguirre et al., 2007). However, shallow neritic environments are inherently prone to site-related effects such as differential subsidence, terrigenous input, shelf profile, substrate conditions, position along the shelf, and coastal upwelling. Despite these limitations, documentation of neritic K/P boundary transitions aids our understanding of the environmental consequences in nearshore habitats during the K/P boundary mass extinction.

In the eastern Alps of Europe, Paleogene synorogenic deep-water clastic units contain calciturbidites and debrites rich in clasts of Paleocene shallow-water limestones (Plöchinger, 1967; Lein, 1982; Tragelehn, 1996; Punekar et al., 2016). This led researchers to postulate that the active orogenic wedge was fringed by a Paleocene carbonate platform that was nearly completely cannibalized and redeposited in deep-water clastic sediments upon synorogenic deformation, uplift, and erosion (Tollmann, 1976; Wagreich and Faupl, 1994; Tragelehn, 1996). However, a shallow neritic K/P boundary interval is preserved on its original substrate only at the Kambühel location (Fig. 1; Plöchinger, 1967; Tragelehn, 1996). To date, the K/P boundary of Kambühel and particularly the sedimentology and biostratigraphy of the Danian interval remain unstudied.

Here, we report on the Kambühel location in the eastern Alps with a specific focus on lithofacies, biofacies, diagenesis, biostratigraphy, and depositional environment marked by a transgressive rocky shore overlain by beachrocks (Fig. 1). In order to better understand the effects of the K/P boundary mass extinction in very shallow platform environments, we compared the deposition and hiatus pattern from Kambühel with similar carbonate platform deposits from Croatia, Oman, Belize, and Guatemala and with nearshore siliciclastic K/P boundary sections from southern Argentina, Tunisia, and Texas. Finally, we compared the results from shallow-water environments with those from deep-sea sections to reveal differences and similarities across the K/P boundary transition from the late Maastrichtian through early Paleocene.

The succession at Kambühel was mapped in light detection and ranging (LIDAR) orthoimages with 1 m isohypse resolution (1 m isohypsed LIDAR orthoimages are commercially available on demand from the government of Niederösterreich, Austria). Rock samples were extracted from vertical sections and from diverse outcrops (Table 1). More than 230 samples were examined in cut slabs, and 220 (4 × 6 cm) thin sections were used to document microfacies, diagenesis, and biostratigraphy. Outcrops are largely confined to low cliffs that fringe the brink of the mesa-like top of Kambühel (cf. Fig. 1C). Farther down section, a few natural exposures and outcrops along road cuts and paths permit thickness approximation of deeper-positioned lithostratigraphic units. LIDAR orthoimages combined with mapped stratigraphic units indicate that the Kambühel hill is cut by faults. Due to insufficient exposure and uncertainty regarding the potential syndepositional relief (see below), the faults could not be restored precisely. For this reason, the thicknesses indicated in the restored summary section (Fig. 2) are approximated. In contrast, the thickness of the Cretaceous-Paleogene boundary section was directly measured from a continuous vertical exposure (outcrop 2 in Fig. 1C; Table 1).

We attempted to deduce the original polymorphy of isopachous fibrous cements that represent widespread initial diagenetic precipitates of lower Danian grainstones. The rationale was that if these cements were originally aragonite, they might have retained elevated Sr concentration; conversely, if they originally were magnesium calcite, an elevated Mg concentration would be expected (cf. Tucker and Wright, 1990; James and Jones, 2016). We carefully selected four samples of well-preserved fibrous cement and tested them with electron probe microanalysis (EPMA). The samples showed no Sr and Mg concentrations beyond analytical uncertainty. EPMA measurements were done on a JEOL JXA-8100 with 5 wavelength-dispersive spectrometers and a SUN-BLADE Workstation with a Solaris (UNIX) operating system and a resolution of ~10 eV. The acceleration voltage was 15 kV, the probe current was 10 nA, and the sample diameter was 20 μm. For Mg determination, the Kα emission line and 50 s measurement time were chosen; for Sr, the Lα emission line and 50 s of measurement time were chosen. Furthermore, element mapping done on two selected thin sections with isopachous fibrous cements (not identical to the thin sections used for EPMA) showed that Sr and Mg were not significantly elevated above background. Element mapping was done in low-vacuum mode on a digital scanning microscope JEOL JSM-5130LV at 15 kV acceleration voltage with an energy-dispersive electron detector, a gun current of 5 μA, and a spot size of 55 μm. The implications of these results are discussed below.

Hereafter, the term “hybrid arenite” designates a grain-supported sediment mainly of sand-sized grains of siliciclastic, carbonate-lithic, and bioclastic composition of highly variable relative mixtures (cf. Zuffa, 1985; Haldar and Tisljar, 2014). In hybrid sandstones, the siliciclastic sediment fraction prevails over the other two fractions.

In the domain of the future eastern Alps, Early Cretaceous stacking of fold-and-thrust nappes was followed by uplift and erosion of the accretionary wedge. From the Turonian onward, the nappe stack was onlapped and overstepped by a synorogenic succession termed the Gosau Group (Wagreich and Faupl, 1994; cf. Platt, 1986), which consists of a lower subgroup of terrestrial to neritic deposits separated by an unconformity from the upper subgroup consisting mainly of deep neritic to bathyal deposits (Wagreich and Faupl, 1994; Sanders et al., 1997). The Northern Calcareous Alps (Fig. 1B), which consist of a nappe stack dominated by Triassic shallow-water carbonates, host large erosional relicts of the Gosau Group. The Late Cretaceous to Eocene depositional setting of the Gosau Group was located at ~30°N–32°N. During Gosau deposition, the eastern Alpine domain was a northward-dipping orogenic wedge crowned by one or more forested islands (cf. Handy et al., 2015). Microtidal shelves were dominated by waves and storms (Sanders, 1998; Sanders and Höfling, 2000). An overall subtropical environment is indicated by buildups of corals and/or rudists (see Darga, 1990; Sanders and Baron-Szabo, 1997; Sanders and Pons, 1999).

In the internal part of the orogenic wedge, metamorphic rocks were exhumed and supplied siliciclastic detritus. Deposition of shallow-water limestones was confined to inner shelves sheltered from terrigenous input. These limestones accumulated from deposystems that consisted of, in a seaward direction: (1) protected shallow subtidal areas (lagoons), (2) bioclastic sand bodies subject to high-energy events, and (3) coral-rudist buildups (Sanders and Höfling, 2000). Another significant limestone deposystem, which includes the Kambühel section, was represented by wave-dominated shore zones struck by episodic high-energy events. The records of these typically transgressive shores are up to ~15 m thick and mainly consist of carbonate lithoclasts mixed with abraded-bioeroded bioclasts of penecontemporaneous marine habitats (e.g., benthic foraminifera with stout tests, bryozoans, rhynchonellid brachiopods, coralline algae, and, rarely, a few rudists and colonial corals). In the Cretaceous part of the Gosau Group, these shore-zone successions coexisted with coral-rudist–dominated deposystems elsewhere (Sanders, 1998; Sanders and Höfling, 2000). Lack of tidal flats of siliciclastic or carbonate type, sedimentological evidence for wave/storm-dominated shore zones, and the tectonic setting on an active margin all suggest that the Gosau shelves were comparatively narrow and steep (Wagreich and Faupl, 1994; Sanders and Höfling, 2000; Wagreich, 2001).

The base of Kambühel hill consists of poorly exposed Lower Triassic siltstones to claystones of the Werfen Formation (Table 2), which unconformably underlie the Piesting Formation of the Gosau Group; the latter formation variably spans from the late Campanian to late Maastrichtian due to syntectonic deposition (Figs. 1C and 2; Table 2; cf. Piller et al., 2004). At the Kambühel hill, the base of the Piesting Formation consists of breccias to shore-zone conglomerates, but most of the formation is a dark-brown–weathered, fossiliferous hybrid sandstone representing neritic environments (brown sandstone unit in Fig. 2; Table 2).

Section 1 up to the top of the brown sandstone unit shows little variation in composition, texture, and bioclasts. The larger benthic foraminiferal assemblage (e.g., Siderolites calcitrapoides, Sirtina ornata) indicates that at least the middle to upper part of the Piesting Formation is of late Maastrichtian age (see Table 2). Above a gap in exposure related to faulting, the brown sandstone unit is overlain by poorly sorted, red-weathered, coarse sandy hybrid arenites with admixed bioclasts from neritic environments (Red arenites in Fig. 2; Table 2). In this unit, abundant intraclasts and strongly fragmented and abraded/bored bioclasts indicate reworking, either during forced regression or during transgressive reworking after a relative sea-level fall (Table 2). No evidence was found to distinguish between these possibilities. In any case, we infer that this unit is underlain by an unconformity (Fig. 2).

The overlying Kambühel Formation consists mainly of limestones that are monotonously light-red to dark-red, indistinctly thick-bedded grainstones and packstones (Red grainstones and packstones in Fig. 2). These limestones are rich in strongly fragmented, abraded, microbored, and encrusted bioclasts (Fig. 3). These largely reworked fossils were probably derived from a transgressive shoreface and were admixed to a parautochthonous neritic bioclast spectrum consisting mainly of larger benthic foraminifera, bryozoans, and planktic foraminifera (Table 2). The oldest part of the Kambühel Formation is exposed in section 2, which contains the K/P boundary (Figs. 1C, 2, and 4). The basal part of section 2 consists of bioturbated packstones rich in late Maastrichtian larger benthic and planktic foraminifera (Schlagintweit et al., 2016). The robust planktic foraminiferal index species Pseudoguembelina hariaensis spans the Upper 191 Maastrichtian zone CF3, equivalent to magnetochron C30n and zones CF2–CF1, equivalent to C29r below the K/P boundary (Keller, 2014). The rest of the planktic foraminiferal assemblage consists of low-diversity, large, complex species with robust shells able to survive in shallow nearshore environments (Figs. 5 and 6; Table 2).

Maastrichtian CF3 limestones are topped by the K/P boundary hardground (Figs. 2 and 4). Above this hardground, reworked Cretaceous foraminifera are common as isolated specimens (typically more-or-less fragmented, abraded, and/or bored) and in clasts. Danian planktic foraminifera are present in the matrix between clasts and occasionally within clasts. The oldest Danian biozone identified is the upper part of zone P1a(1) (biozonation by Keller et al., 1996, 2002), which spans from the first appearance of Parvularugoglobigerina eugubina to the first appearances of Parasubbotina pseudobulloides and/or Subbotina triloculinoides (Fig. 5). The major part of the Kambühel Formation consists of Danian limestones (Figs. 1C and 2).

The most widespread facies consist of grainstones that became cemented by an isopachous fringe of fibrous cement, or micritic meniscus and pendant cements. Typical components include coralline algae, bryozoans, common rhynchonellid brachiopods (e.g., section 2), intraclasts, and lithoclasts (Table 2). About 2 m above the K/P boundary hardground (section 2), a more diverse assemblage of 10 identified planktic foraminiferal species marks zone P1a(2), with the top of this zone identified by the last appearance of P. eugubina, which is equivalent to the top of magnetochron C29r (Fig. 6). Above this interval, there is a second Danian hardground that contains an impoverished and poorly preserved planktonic foraminiferal assemblage of zone P1b, equivalent to C29n (Figs. 4A and 6). Above the intra-Danian hardground, only a few planktic foraminifera are present, including the index species Praemurica inconstans, which marks the upper half of zone P1c(2). This suggests dissolution, nondeposition, and abrasion effects, as also indicated by a persistent content in reworked Cretaceous species up to the hardground (Figs. 4A and 5).

At two locations, Danian limestones are overlain by a few meters of grainstones deposited during the ?Selandian–Thanetian interval (Figs. 1C and 2). The bioclast content of the grainstones is similar to those of Danian limestones, but large fragments of colonial corals and the red alga Parachaetetes are also common (Fig. 2; Table 2). Along the base of the ?Selandian–Thanetian unit no clear-cut boundary surface or sharp facies change was identified. Near the southeastern corner of the Kambühel hill, a unit of floatstones, bafflestones, and rudstones is exposed with branched corals, massive and foliose corals, Parachaetetes, coralline algae, and hydrozoans (Fig. 1C; coralgal limestones in Fig. 2). The matrix consists of bioclastic wackestone to packstone with smaller benthic foraminifera (Miliolina, Textularina, Rotaliina), fragments of diverse dasycladaleans, and planktonic foraminifera. Globorotaliids allow for identification of the Upper Thanetian (P4c biozone; Table 2).

Sediment textures and fossils in the coralgal limestones record deposition in shallow subtidal, normal-marine environments struck by episodic high-energy events (Table 2). The coralgal limestones unit most probably overlies an unconformity, as suggested by the different facies contents and late Thanetian age. The entire Lower Danian to Upper Thanetian succession is no more than a few tens of meters thick (Fig. 2) and consists entirely of shore-zone to protected shallow-subtidal facies, which include at least two hiatuses, with the missing intervals fairly well constrained. This indicates that the Kambühel Formation is rich in stratigraphic gaps related to intermittent subaerial exposure and sea-level fluctuations.

The ages and durations of hiatuses and hardground formation can be estimated based on correlation of biostratigraphy (biozone ages) with magnetostratigraphy and cyclostratigraphy calibrated with U-Pb geochronology. High-precision U-Pb zircon dating of the Deccan Traps in India yielded a 65.552 ± 0.026 Ma age for the top and 66.288 ± 0.027 age for the base of C29r, for a total duration of 736 ± 37 k.y. (Schoene et al., 2015). The K/P boundary is currently placed at ca. 66.021 ± 0.024 Ma, based on U-Pb dating in the Denver Basin, Colorado (Clyde et al., 2016), and recalculated at 65.970 ± 0.030 Ma. This suggests that C29r below the K/P boundary spans a maximum of 345 k.y., which is within the error margin of the ~350 k.y. derived from orbital cyclostratigraphy estimates in marine sections (Sinnesael et al., 2016; Thibault and Husson, 2016; Thibault et al., 2016). Differences in cyclostratigraphy of marine sections are generally due to the number of cycles present (16 vs. 18), with the lower number likely due to K/P boundary section erosion, as suggested by the frequent absence of the topmost Maastrichtian rapid global warming that spans ~40 k.y. or two cycles (Thibault et al., 2016).

The duration of planktic foraminiferal zones CF1 and CF2 spans from the upper C30n magnetochron to the K/P boundary. Their ages based on cyclostratigraphy of the auxiliary stratotype section at Elles, Tunisia, reveal that zone CF1 spans the last 200 k.y. of the Maastrichtian ending at the K/P boundary, whereas zone CF2 spans the preceding 250 k.y. (Thibault et al., 2016). This entire interval is missing at Kambühel below the K/P boundary. In the early Danian, zones P0, P1a(1), and P1a(2) correspond to C29r above the K/P boundary, spanning ~420 k.y. (65.552 ± 0.026 Ma to 65.970 ± 0.030 Ma). At Kambühel, the missing interval spans zones P0 and most of P1a(1), or about ~150 k.y. The K/P boundary hiatus thus spans at least 600 k.y.

The preserved Danian part of the Kambühel Formation accumulated during an interval of time corresponding to the upper part of P1a(1) into zone P1b (Figs. 5 and 6). Age estimates for the overlying intra-Danian hardground (Figs. 4 and 6) can be derived from correlation of biozones to magnetostratigraphy (Olsson et al., 1999; Berggren and Pearson, 2005). Based on this age scheme, the hiatus spans from the upper zone P1b into the upper zone P1c(2), or about ~1.5 m.y. (Fig. 6; Table 2). This implies that the maximum age of the Danian interval represented at Kambühel is ca. 3.2 Ma, and at least half of this interval is missing due to nondeposition and erosion. Another Paleocene hiatus is inferred toward the topmost late Thanetian part (Coralgal limestones) of the Kambühel Formation (Figs. 1 and 2; Table 2).

At Kambühel, Upper Maastrichtian limestones below the K/P boundary are gray- to red-colored, bioturbated bioclastic packstones with a wide spectrum of bioclasts from neritic environments rich in benthic foraminifera (Table 2). Many bioclasts are broken, abraded, and bioeroded (Fig. 3). In addition, rounded clasts of light-brown siltstone to claystone from the underlying Lower Triassic Werfen Formation are common. Backscattered-electron element mapping indicated that the typical brown stain of the clasts is related to iron, probably iron oxides (goethite, limonite). Maastrichtian limestones are riddled with a system of cavities varying from 1 mm to a few centimeters in width and at least 6–12 cm in vertical extent (height of polished rock slabs; Fig. 7). Locally, the margins of the cavities are coated by a thin crust of dark-brown iron oxide (Fig. 7D). In the centimeter-thick margin surrounding the cavities, the host packstone is riddled with open pores between bioclasts. These pores commonly show complex, variable infillings, including (1) geopetal fabrics of lime mudstone, often in several layers separated by red- or brown-stained surfaces of irregular shape, (2) isopachous fringes of cement, (3) crystallized silt, and (4) micropeloidal grainstones (Fig. 7D).

The cavities are most commonly filled with poorly sorted Lower Danian litho-bioclastic grainstones to packstones rich in bryozoan fragments, with rhynchonellid brachiopods, and with common intraclasts of Lower Danian bryozoan limestones. Bioclasts and intraclasts range in preservation from fresh and unaltered to more commonly abraded, rounded, microbored, and/or encrusted. Lower Danian limestones in cavities are riddled with fenestral pores, vugs, and fractures that show complex infillings (e.g., cement fringes, geopetally laminated lime mudstone, grainstone). In addition, geopetally laminated, red to pink lime mudstone to lithic wackestones to packstones show infillings occasionally cut by irregular dissolution surfaces, followed by renewed infillings of fine-grained carbonate sediments.

A hardground with complex morphology separates the Upper Maastrichtian and Lower Paleocene Danian sediments (Fig. 8). The hardground surface is marked by delicate millimeter-scale projections excavated from Upper Maastrichtian bioclastic packstone, as well as cuspate and convex-concave projections and recesses ranging from 1 mm to a few centimeters in width; deeper recesses of the hardground are overlain by Danian packstone (Fig. 9). Up section and along higher-projecting intervals of the hardground, the packstone is overlain by Danian grainstone rich in coralline-algal and bryozoan fragments. In polished slabs and thin sections, the hardground shows a vertical relief of ~3 cm (Fig. 10). However, the local vertical relief in the field ranges up to a few decimeters over a lateral distance of a few meters. Rock samples from similar elevation in the K/P boundary interval consist of Maastrichtian limestones (some riddled with cavities), exposed K/P boundary hardground (two samples), and Lower Danian limestones (Figs. 4B and 7). This indicates a highly irregular hardground surface.

Lower Danian limestones range from very poorly to well-sorted grainstones to fine-grained rudstones (see Table 3A). Many grainstones show fenestral pores and/or characteristic sparse packing of components. Grainstones mainly consist of: (1) abraded and microbored fragments of bryozoans and corallinaceans, vertically and laterally poorly delimited levels rich in the rhynchonellid Basilicocostella (Dulai et al., 2008), and, to a lesser extent, smaller benthic foraminifera (rotaliines, miliolids) and planktic foraminifera; (2) lithoclasts derived from older substrate rocks (Werfen Formation; Table 2); and (3) first- and second-cycle intraclasts frequently consisting of bored and/or encrusted grainstones to packstones with components identical to the Danian host grainstone or Upper Maastrichtian limestone (Fig. 11). The bryozoan fauna consists of Cheilostomata and Cyclostomata, with representatives of both groups grown in erect positions and delicately branched, which probably required hard substrate for attachment (B. Berning, 2016, personal commun.). Grainstones consist almost exclusively of skeletal fragments of low-magnesium calcite (bryozoans, foraminifera, rhynchonellids) or magnesium calcite (red algae, echinoderms). Bioclasts that originally consisted of aragonite, such as calcareous green algae, bivalves, and gastropods, are very rare and only preserved as biomolds filled with later diagenetic products.

Diagenetic sequences of the grainstones are quite uniform and subdivided into two types (Table 3B). Type A sequences mostly initiated with dissolution of aragonitic bioclasts (e.g., gastropods), followed by precipitation of an isopachous fringe of radial-fibrous cement, whereas remnant pore spaces, if present, are filled with blocky calcite spar (Fig. 11).

In type B sequences, dissolution of aragonitic bioclasts was followed by geopetal infilling of lime mudstone and/or micropeloidal grainstone into biomolds and interstitial pores. In addition, sediment grains are bound by micritic meniscus and/or pendant cements; together, these micritic cements typically occur in patches and roughly stratiform lenses (Fig. 12). The infilling of internal sediments was locally interrupted by dissolution, resulting in micro-hardgrounds that vertically separate discrete phases of infill. The remaining pore space is filled with an isopachous fringe of radial-fibrous cement and/or blocky calcite spar (Fig. 12; Table 3B). Rock samples of primary packstone texture are rare. Sediment sorting in these packstones is identical to the more widespread grainstones. Packstones show dissolution vugs that may be entirely or partly filled with sediment and/or cements or micritic to micropeloidal sediment of probable bacterial origin.

From the top of planktic foraminiferal zone P1a to the top of section 2, the nature of sedimentation remains constant and marked by well-sorted to poorly sorted bioclastic grainstones (rarely packstones) with fenestral pores, partly or entirely lithified by isopachous fringes of fibrous cement. In the upper part of the section, a hardground sharply separates a packstone from overlying packstones and grainstones (Fig. 13). The packstone beneath the hardground is intercalated into a succession of grainstones with fibrous cements and is riddled with multiple macroborings filled with reddish carbonate-lithic wackestone. Furthermore, the packstone is riddled with vugs filled with: (1) geopetal fabrics of lime mudstone to calcisiltite, and (2) a translucent cement with a crystal size too small to be clearly resolvable. The hardground is overlain by packstone and grainstone rich in intraclasts with packstone texture. Up section, only grainstone with fibrous cement was observed. Sampling of shorter outcrop exposures that are laterally correlative with larger outcrops indicated similar facies characteristics, at least for zones P1a to P1b (Fig. 6). These Danian sediments underlie bioclastic grainstones and packstones of ?Selandian to Thanetian age (Fig. 2) with similar facies and bioclast characteristics as the underlying Danian interval, except for the presence of larger fragments of corals and Parachaetetes.

Age dating of the beachrock sequence across the K/P boundary transition at Kambühel is difficult because of its sparse and sporadic foraminiferal influx. Nevertheless, it was possible to estimate that the surface of the K/P boundary hardground represents a period of ~600 k.y. of nondeposition and erosion, which is similar to that of the deeper slope succession at Gamsbach in the eastern Alps (Punekar et al., 2016). The change from late Maastrichtian siliciclastic shelf deposition to terminal Maastrichtian–Paleocene shallow-water limestones (see Figs. 2 and 3) was not confined to beachrocks of the Kambühel area. In bathyal to abyssal successions deposited along the slope of the orogenic wedge, beds of carbonate-lithic turbidites and megaliths up to hundreds of meters in size record a Paleocene carbonate platform, which was cannibalized by synorogenic uplift and erosion (Tollmann, 1976; Wagreich and Faupl, 1994).

Biostratigraphic data indicate that platforms persisted during the Paleocene, but may have been localized (Tragelehn, 1996). Similarly, Eocene neritic limestones are rarely preserved in place (Van Hinte, 1963; Darga, 1990; Rasser, 1994), and most records are from clasts reworked into molasse deposits (e.g., Hagn, 1972, 1989; Moussavian, 1984; Rasser, 2000). Due to progressive Late Cretaceous to Paleogene onlap onto the accretionary wedge (Wagreich and Faupl, 1994), potential provenance areas for siliciclastics were overstepped, which favored the establishment of shallow-water platforms. This suggests that deposition of Paleocene to Eocene shallow-water limestones was widespread in the eastern Alps, but determining the precise extent is elusive (Nebelsick et al., 2005). The Paleogene resurgence of neritic carbonate deposition over large parts of the Alps was probably related mainly to tectonism rather than climatic aridification.

Bioperforations, infillings of diagenetic cavities, and diagenetic alterations of sediments yield important information about seafloor occupants, substrate conditions, and postdepositional overprint. In the Kambühel area, Maastrichtian limestones underlying Lower Danian limestones are marked by vugs and cavities with haloes of diagenetic grainstones. These cavities probably formed during subaerial exposure, whereas their multiphase infillings of red-colored mudstone and/or of grainstones resulted from the subsequent marine transgression. During subaerial exposure, the Kambühel area probably was a karstic terrain covered by soil. Upon transgression, the soil was stripped, and a rocky shore—the K/P boundary hardground—developed (Fig. 14). The complicated relief of the bored and encrusted hardground resulted from physico-chemical and biological processes in the shallow subtidal to supratidal zone of a low-energy rocky shore (cf., e.g., Trudgill, 1976; Laborel and Laborel-Deguen, 1996; Spencer and Viles, 2002; Twidale et al., 2005; Moura et al., 2006). Field mapping and biostratigraphy indicate that a steep relief of substrate rocks (e.g., buried cliffs or karst towers) is absent in the Kambühel area. Despite the potentially complex microscale morphology, this transgressive shore is reminiscent of a shore platform (cf. Moura et al., 2006).

The radial-fibrous cements in the Danian grainstones most probably precipitated from supersaturated seawater pumped through the pore space of sandy to fine-pebbly sediment. Absolute concentrations determined by EPMA yielded no conclusive information about the origin of the cement (see Methods section). We attribute this to repeated meteoric-diagenetic overprinting during episodes of subaerial exposure. Meteoric diagenesis is recorded by presence of cayrocks in the section and probably by the hardground identified in the Lower Danian succession. Overprint of fibrous cements, most probably by meteoric diagenetic processes, is underscored by their highly variable preservation even within single thin sections.

We saw no evidence that the radial-fibrous cements originated in a terrestrial setting far landward of the shoreline, or in a deep (few tens of meters) neritic setting. In the former case, a strong meteoric-vadose overprint of the grainstones would be expected, which contradicts the pervasive radial-fibrous isopachous cementation. In the latter case, intercalated intervals of deep neritic deposits (e.g., marls with planktonic foraminifera) would be expected. Isopachous radial-fibrous cements are typically of marine origin and result from percolation of seawater supersaturated with aragonite and (magnesian) calcite through the sediment pore space (e.g., Bathurst, 1975; Davies, 1977; Tucker and Wright, 1990; Tobin and Walker, 1996; James and Jones, 2016). Isopachous cement of fibrous aragonite may form even in meteoric-vadose settings, yet such cements are confined to patches a few centimeters wide (Ostermann et al., 2007). Notwithstanding the original polymorphy (or polymorphies) of cements, the pervasive isopachous radial-fibrous cementation is best interpreted as of shallow-marine origin.

Because of their initial lithification by isopachous radial-fibrous cements of shallow-marine origin, the grainstones represent beachrocks (cf., Bathurst, 1975; Flügel, 2004; Vousdoukas et al., 2007; Friedman, 2011). These may be subdivided into: (1) beachrocks sensu stricto, with marine-isopachous cement as the main first precipitate, and (2) cayrocks with micritic cements formed under vadose conditions as the major early diagenetic product (see, e.g., Bathurst, 1975; Gischler and Lomando, 1997; Friedman, 2011; Erginal et al., 2013; Christ et al., 2015). Because the entire Danian interval in section 2 (Fig. 4) consists mainly of beachrocks and cayrocks, this interval probably consists of stacked units separated by diastems and/or hiatuses (cf., e.g., Cooper, 1991; Vousdoukas et al., 2007; May et al., 2012; Erginal et al., 2013). A stacking of beachrock/cayrock layers requires net relative sea-level rise, albeit interrupted by nondeposition and erosion (Fig. 14). The abundant evidence for abrasion, bioerosion, and encrustation of bioclasts before final embedding records longer exposure on the seafloor at a low rate of sediment accumulation.

Macrofossil and microfossil assemblages yield important information about the depositional environment, including nutrients, salinity, oxygen, and temperature. For example, an assemblage of bryozoans, red algae, and brachiopods may indicate cool-water and/or high-nutrient conditions (e.g., Carannante et al., 1988; James et al., 1992; Bone and James, 1993; Boreen et al., 1993; Vecsei and Sanders, 1999; Jaramillo-Vogel et al., 2016). In the Lower Danian limestones at Kambühel, bryozoans may reflect the cooling after the K/P boundary event that is known from stable isotopes of benthic foraminifera at Elles, Tunisia, and elsewhere (Stüben et al., 2003, 2005). Some workers attributed this cooling to the aftereffects of the Chicxulub impact (Galeotti et al., 2004; Vellekoop et al., 2015). However, the long-term nature of this cooling, concurrent delayed biological recovery and coincident with Deccan volcanism through the early Danian C29r and into C29n, strongly points to Deccan volcanism–related climate change (Keller et al., 2011a, this volume; Punekar et al., 2014). At Kambühel, the preserved Lower Danian sediments rich in bryozoans above the K/P boundary hardground were deposited in the upper zone P1a(1) (planktonic foraminifera) at least 150 k.y. after the K/P boundary event.

Articulate brachiopods flock on hard substrates, such as flanks and shady overhangs of cliffs or boulders. Intervals with abundant articulate brachiopods probably accumulated during storms (cf. Sedgwick and Davis, 2003; Marriner et al., 2017) and/or tsunamis (cf. Donato et al., 2008; Massari et al., 2009), which may have contributed to beachrock aggradation (cf., Morton et al., 2007; Wang and Horwitz, 2007). Similarly, coralline algae and bryozoans prefer hard-substrate habitats (cf., e.g., Betzler et al., 2000; James and Bone, 2011; James et al., 2013). Because beachrock formation steadily modifies the shore zone in favor of cemented substrate (Cooper, 1991; Vousdoukas et al., 2007), this may help to explain the persistence of hard-substrate biota in the Lower Danian section at Kambühel.

Extreme scarcity of colonial coral fragments in the Lower Danian section of the Kambühel limestone is probably unrelated to the mass extinction. Reef corals suffered only modest diversity loss across the K/P boundary transition (Kiessling and Baron-Szabo, 2004), and coral reefs grew again during the Danian (e.g., Vecsei and Moussavian, 1997; Vecsei et al., 1998; Baceta et al., 2005). In the Pyrenean Basin, shelf-edge coral reefs coexisted with foreslopes rich in bryozoans and red algal fragments (Baceta et al., 2005; Aguirre et al., 2007). At Kambühel, the common presence of the encrusting acervulinid foraminifer Solenomeris (Schlagintweit et al., 2018) suggests that the scarcity of corals may be explained by other factors. Comparative analysis of Solenomeris in Danian and Ypresian strata of the Pyrenees indicated that this species was resilient to siltation and lower illumination, which would adversely affect corals and coralline algae (Plaziat and Perrin, 1992). Solenomeris may have bloomed at Kambühel due to reduced competition with corals and corallines related to transgressive soil reworking and an overall higher resilience to unstable habitat conditions (Schlagintweit et al., 2018).

The depositional setting of the Gosau Group thus was composed of shelf sectors with coral reefs coexisting with rocky shores characterized by bryozoans, coralline algae, and brachiopods (for similar examples in the Miocene, see Moissette et al., 2007; Reuter et al., 2012). As mentioned, the total sedimentological evidence indicates that the Gosau shelves were relatively narrow and steep. Such shelves are vulnerable to coastal upwelling. Thus, at least the shelf sectors fringed by rocky shores may have been episodically impinged by cooler, nutrient-rich waters. Together, the evidence suggests that the Lower Danian bryozoan–red algal–brachiopod assemblage of Kambühel was not related to coral extinction or to exceptional oceanographic conditions, but rather resulted from normal processes in the given depositional setting—a transgressive shore rich in hard substrates and associated biohabitats.

The Kambühel stratigraphic and hiatus records are complex, and viewed in isolation, they could be seen as simply local phenomena of little or no consequence to the overall history of sedimentation and depositional environments across the K/P boundary. However, when viewed within the global stratigraphic context of late Maastrichtian to Early Paleocene sedimentation and hiatus patterns, this shallow carbonate platform and rocky shore yield important information about the environmental effects at the continental margins. To elucidate this little-known environment during the K/P boundary transition, we briefly review and illustrate the global stratigraphy and erosional patterns of shelf to deep-marine environments. We then compare this record with Kambühel and other known marginal marine platform carbonates and siliciclastic sequences with the objective to place them within the larger historical context of the K/P boundary mass extinction event.

The K/P boundary transition has been documented in over 300 sections worldwide, from continental shelf to deep-water environments that largely yield excellent stratigraphic records with high-resolution age control based on diverse and abundant planktonic foraminiferal and nannofossil assemblages (reviews in Keller, 2008, 2011b; Keller et al., 2013; Punekar et al., 2014; Thibault et al., 2016). The most complete K/P boundary records are known from shelf to upper-slope environments in Tunisia, particularly, the El Kef K/P boundary stratotype, although this section has a tectonically disturbed Maastrichtian section (Keller, 1988; Keller et al., 1996, 2002; Li and Keller, 1998a; Li et al., 2000; Keller et al., 2016a). In contrast, the auxiliary stratotype Elles section, located 75 km to the northeast and deposited on the continental shelf, has undisturbed sedimentation with high terrigenous influx (e.g., Abramovich and Keller, 2002; Keller et al., 1996, 2002; Molina et al., 2006; Thibault et al., 2016). Nonetheless, high-resolution quantitative faunal, stable isotopic, and cyclostratigraphic age controls reveal that even these two stratotype localities have short hiatuses in the early Danian, similar to localities in Egypt, Israel, and Spain (Punekar et al., 2014, 2016; Font et al., 2016, 2017). During the Maastrichtian, deep-sea hiatuses are relatively few and erosion is limited at the C31r/C31n and C30n/C29r transitions and within C30n (Fig. 15).

Deep-sea hiatuses generally correlate with climate cooling, sea-level falls, intensified oceanic circulation, and Deccan volcanism (Fig. 15), as supported by the fact that similar early Danian hiatuses are observed in marine sequences worldwide (Keller et al., 2016a, 2016b; Mateo et al., 2017). However, the extent of erosion is variable. In sequences with high sediment accumulation rates on continental shelves and upper slopes (e.g., Tunisia, Egypt, Israel, Spain), erosion is relatively limited. However, on topographic highs, such as mid-ocean ridges (e.g., Ninetyeast Ridge DSDP Sites 216, 217], Walvis Ridge [Site 525]), and uplifted areas of southern India (Cauvery Basin) and Madagascar (due to doming associated with Deccan volcanism), erosion is extensive in the early Danian and may encompass most or all of the early Danian and upper part of the Maastrichtian (see refs in Fig. 15). Additional localities with similar erosion patterns include the Caribbean Plateau (Sites 999B, 1001), North Atlantic Blake Nose (Sites 1049, 1050), and New Jersey (Bass River; Keller et al., 2013; Mateo et al., 2016, 2017).

Shallow-neritic K/P boundary sequences and associated biotic turnovers are relatively poorly documented, particularly in carbonate platform settings, which generally include major hiatuses spanning from several hundred thousand to millions of years (e.g., Guatemala, Belize, Oman, southern Mexico; Keller et al., 2003a, 2003b; Stinnesbeck et al., 1997; Ellwood et al., 2003, for Oman; see Fig. 16). Similarly, hiatuses are common in shallow nearshore areas or inland basins with high siliciclastic input, such as Argentina, Brazil, southern Tunisia, Denmark, and Texas (Fig. 16; Keller et al., 1997, 2007, 2011a, 2011b; Gertsch et al., 2013). The stratigraphic extent of these hiatuses is variable and in many cases uncertain due to low species diversity and rarity or absence of index microfossils.

In the western Tethys domain, shallow-water K/P boundary transitions are reported from the northern margin of the Apulian Platform (Vecsei et al., 1998), the Adriatic Platform (Drobne et al., 1996; Caffau et al., 1998; Korbar et al., 2015, 2017), and from locations in Turkey (Sirel, 1998, 2015; Inan and Inan, 2014). These carbonate platform sections represent shelfal to reefal environments and/or internal platform settings. In the Pyrenean Basin, absence of coral reefs during the early Danian is related to the depositional setting rather than the extinction of hermatypic corals, because coral reefs return during the later Danian (cf. Baceta et al., 2005; Aguirre et al., 2007).

In the eastern Alps, the Gamsbach valley section of Austria is a deep-water analog of the rocky shore Kambühel section. At this locality, the K/P boundary is documented from an upper-slope sequence (500–1000 m water depth) where the K/P boundary hiatus spans part of the latest Maastrichtian zone CF1, plus zone P0, part of zone P1a(1), and part of zone P1a(2) (Punekar et al., 2016), for an estimated total of ~500 k.y. (Fig. 16). This hiatus thus spans the mass extinction of all but the disaster opportunist Guembelitria cretacea, the sole long-term survivor. The top of this hiatus is marked by the simultaneous appearances of 14 new early Danian species that are known to have evolved sequentially during the early Danian—zones P0, P1a(1), P1a(2). There is no K/P boundary clay (P0), which is confirmed by the abrupt shift in δ¹³C values from Maastrichtian to early Danian (Grachev et al., 2005; Punekar et al., 2016).

Carbonate platform environments, such as Kambühel, are relatively common but rarely studied in depth and with good age control. For comparison with Kambühel, we chose similar environments from Croatia, Oman, Madagascar, Belize, and Guatemala (Fig. 16). Hiatuses were identified based on erosion surfaces, hardgrounds, and karstification. Age control was based on characteristic foraminifer biozone assemblages, which are frequently absent or only partly represented. From the emerging pattern of preserved partial biozones, it is clear that the hiatus distribution is very similar in all these geographically widespread areas, indicating a global imprint, even though the extent of erosion is variable (Fig. 16). Erosion was most extensive during the early Paleocene in the early part of magnetochrons C28n to late C29n, early Danian C29r to C29n, and late Maastrichtian C29r and C30n. The few records available for the early and late Maastrichtian also show correlative hiatuses. The most significant feature for the K/P boundary transition is the absence of the mass extinction interval in all carbonate platform sections, generally due to hardgrounds and/or karstification. In most cases, a partial record of the early Danian C29r is present (partial zones P1a[1] and/or P1a[2]) and a partial record of C29n is present (partial zone P1b]; Fig. 16). Thus, only a small part of the first ~400 k.y. of early Danian C29r (zones P0, P1a[1], P1a[2]) is present in any shallow carbonate platform environment. Moreover, erosion and/or nondeposition spans C29r below the K/P boundary and into C30n (CF1-CF2) for ~500 k.y. Only rarely is this interval partially present (e.g., Madagascar).

We also compared platform carbonates with shoreface siliciclastic deposition of inland seas with high terrigenous inputs, such as in Egypt, Tunisia, Argentina, and Texas (Fig. 16). Except for Texas (Brazos River), the K/P boundary is also absent in these environments, and early Danian erosion removed also the latest Maastrichtian record. This may have been due to lower terrigenous input, sea-level fluctuations, seasonal storms, and/or regional uplift, which may have acted in concert to amplify erosion of underlying sediments.

A rare exception is the Brazos River paleoenvironment of Texas, where a dozen outcrops and cores reveal a shallow nearshore environment marked by local topographic highs and associated variable erosion patterns. This environment at the shore of the Western Interior Seaway was tectonically stable with high terrigenous influx and boasts the most complete shallow-water sequence known to date (Adatte et al., 2011). The Brazos River environment shallowed from middle neritic depth during the late Maastrichtian zone CF2 to inner neritic depth with a sharp sea-level drop in CF1. In the deeper inner neritic parts, the sea-level fall was accompanied by erosion, which carved submarine channels that infilled with shore debris, shell hash, glauconite, and Chicxulub impact spherules (Keller et al., 2011b). Normal deposition continued above these channel deposits and preserved the K/P boundary transition as well as most of the early Danian zones P0 and P1a(1), followed by another hiatus between P1a(1) and P1b (Fig. 16). However, on local topographic highs (Darting Minnow Creek), only the channel deposit is present, followed by zone P1a(2). In a core drilled just 150 m from Darting Minnow Creek, an unconformity marks a mangrove root level in zone CF1, channel deposits are absent, and sedimentation resumed in zone P1a(2) (Fig. 16; Adatte et al., 2011).

Brazos River sections thus reveal the different expressions of sea-level change and resulting patterns of sediment dispersal and erosion in relation to variable topography. Nevertheless, the hiatus distributions of neritic environments generally reveal the same pattern as in deep-water environments. We conclude that despite very different depositional settings in platform carbonates, shoreface siliciclastic, and deep marine environments, hiatus patterns indicate the same underlying causes at work (e.g., sea-level fall, subaerial to submarine erosion, climate cooling) in shallow- and deep-sea environments, even though erosion is generally more extensive in shallow-water settings (Figs. 15 and 16).

1. In a synorogenic succession of Upper Maastrichtian to Thanetian shallow-water limestones (Kambühel Formation) of the eastern Alps, the K/P boundary is marked by a hardground and a hiatus that juxtaposes late Maastrichtian zone CF3 (Pseudoguembelina hariaensis) and early Danian (upper P1a[1] zone), indicating an ~600 k.y. gap due to nondeposition and erosion.

2. The Upper Maastrichtian part of the succession consists of bioturbated packstones rich in larger benthic foraminifera (e.g., Orbitoides, Lepidorbitoides, Siderolites, Clypeorbis) and planktic foraminifera (e.g., Pseudoguembelina, Planoglobulina). These limestones are riddled with (micro)karstic cavities near their top, and, particularly along the margins of dikes, they are transformed by dissolution of interstitial matrix into diagenetic grainstones. Dikes are filled with Lower Danian bio-lithoclastic grainstones to rudstones rich in fragments of coralline algae, bryozoans, and clasts derived from Maastrichtian limestones and Triassic rocks.

3. The K/P boundary hardground reveals a bizarrely cusped relief upon Maastrichtian limestones penetrated by borings and/or encrusted by sessile organisms (e.g., foraminifera, serpulids). Overlying Lower Danian deposits are poorly sorted bio-lithoclastic grainstones with fenestral pores and isopachous fringes of radial-fibrous cement. Deposition during the early Danian transgression occurred on a rocky low-energy shore that experienced episodic high-energy events.

4. Another hardground in the Upper Danian is overlain by beachrocks and cayrocks characterized by fragments of bryozoans (cheilostomes, cyclostomes), red algae, lithoclasts from older rocks, and intraclasts. The presence of planktic foraminiferal assemblages indicates a period of nondeposition and erosion that brackets a hiatus of ~1.5 m.y. Deposition occurred in persistent hard-substrate conditions on a rocky shelf, as indicated by the bryozoan–coralline algal composition of the beachrocks–cayrocks.

5. In the Upper Thanetian, floatstones to bafflestones with colonial corals, solenoporaceans (Parachaetetes), foliose corallines, milioline foraminifera, and dasycladalean algae record the establishment of an open, shallow subtidal habitat with lime-muddy substrate.

6. We compared the Kambühel shallow-water K/P boundary transition with similar shallow-water transitions globally and with the global deep-sea record. The comparison reveals very similar patterns in shallow- and deep-water environments, although erosion is more extensive and sequences are less complete in shallow waters due to sea-level falls and potential local tectonic activity. We conclude that climate and sea-level changes were the driving forces for the observed sedimentation and erosion patterns and that climate change across the K/P boundary transition (C29r-C29n) was mainly driven by Deccan volcanism.

Financial support from grant “Nachwuchsförderung der Universität Innsbruck,” project W715505 (to M. Studeny), is gratefully acknowledged. Martina Tribus, Institute of Mineralogy and Petrography, is thanked for her help and expertise in electron probe microanalytical measurements as well as in element mapping. Björn Berning, Biologiezentrum-Oberösterreichisches Landesmuseum Linz (Austria), is thanked for taxonomic affiliation of bryozoans in thin sections. The constructive comments by an anonymous reviewer and by Annie Arnaud-Vanneau improved the clarity of presentation. The University of Innsbruck is thanked for donating financial support for Gold Open Access publication.