Onlap of Upper Cretaceous synorogenic wedge-top deposits (Eastern Alps): Fragmentites, event beds, and coseismic deformation.

doi:10.21203/rs.3.rs-4211447/v1

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Onlap of Upper Cretaceous synorogenic wedge-top deposits (Eastern Alps): Fragmentites, event beds, and coseismic deformation.

https://doi.org/10.21203/rs.3.rs-4211447/v1

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In synorogenic wedge-top basins of the Alps, the basal strata are rarely exposed. Herein, a late Turonian to early Coniacian succession above an older rock substrate is described that comprises: (a) proto-/cataclasites, (b) slope breccias, (c) fluvial conglomerates, and (d) low-energy lagoonal limestones with event beds. The proto-/cataclasites are a vestige of a fault damage zone. The distribution of the other facies records a hilly vegetated terrain drained by a stream and that, upon sea-level rise, became encroached by a lagoon. A section of 'lagoonal' limestones contains a thick event bed of angular dolostone pebbles to cobbles derived from the local substrate, and embedded in hybrid arenite. Bioclasts (rudists, corals, red algae) are from open-marine environments. Angular shape and a corrugated surface of lithoclasts suggest that they stem from slope mantles of subaerially exposed areas. The bed records an event of exceptional coastal flooding and is tentatively interpreted as a tsunami deposit. In the slope breccias, the fluvial conglomerates, and in the event beds, many lithoclasts became fractured in situinto crackle, jigsaw, and mosaic subclast fabrics; fractures are filled with the same material (lime mud to arenite) as that comprising the primary matrix. In-situ clast fracture was produced by coseismic shaking, i.e., these fabrics record syndepositional earthquakes. In the Meso-Cenozoic wedge-top succession of the Eastern Alps, features related to seismic activity and coastal inundation expectably are widespread, but to date were overgone or not identified as such.

Cretaceous

Alps

Gosau Group

onlap

inundation

coseismic deformation

Since the 19th century, the 'Gosau deposits' or the Gosau Group (middle Turonian to Ypresian; Wagreich and Faupl 1994) is known as the synorogenic wedge-top succession of the Eastern Alps. In consequence of a geohistory including stacking of thrust nappes, uplift and subaerial erosion, repeated transgressions over a differentiated morphological relief and, finally, deepening to bathyal to abyssal depths, the Gosau Group comprises an exceptionally wide spectrum of facies (e.g., Wagreich and Faupl 1994; Sanders 1998; Wagreich 1998; Sanders and Höfling 2000; Sanders et al. 2019; Kearney et al. 2022; Koukal et al. 2022; Sanders and Gruber 2023). The lowermost portions of the onlapping Gosau successions, however, are only rarely well-exposed because they are commonly covered by scree, vegetation or glacigenic sediments. In addition, at many locations, the unconformity between older rocks and overlying Gosau deposits is not easy to pinpoint due to presence of intervals of monomictic, substrate-derived, carbonate-lithic fragmentites of uncertain stratigraphic affiliation: Do these still pertain to the substrate or are they already basal Gosau? Indeed, such fragmentites had given rise to conflicting interpretations where to place the boundary. So far no study has been made to identify the nature of the fragmentites and their association with other basal deposits.

Another feature related to the high seismicity of active margins are events of coastal inundation due to tsunami impact (e.g., Spiske and Bahlburg 2011; Spiske et al. 2014; Ryan et al. 2015; Takashimizu et al. 2020). Although tsunami-related inundation is well-recorded in shallow offshore settings, transient coastal submergence is also accomplished by storms, such that it is not straightforward to distinguish tsunami-related event deposition from that produced by a storm (e.g., Shanmugam 2012; Paris et al. 2018). Herein, an event bed is described that is tentatively interpreted as a tsunami-related deposit. Despite the wedge-top setting, records of palaeoseismicity such as coseismic rock fracture (e.g., neptunian dykes filled with multi-phase breccias) and soft-sediment deformation structures so far have been rarely described from the Gosau Group (Sanders 1998, 2001), but should be more widespread as pretended by scarcity of literature. Herein, brittle fracture of lithoclasts and deformation bands in paralic deposits are described that formed while the sediment was still in a soft state, supporting an interpretation in terms of syndepositional coseismic deformation.

In the study area (Figs. 1, 2), excavation of roadcuts and timbering in 2014 provided outcrops of the basal part of a transgressive Gosau succession which is notable in that: (a) it provides an example for a low-energy marine transgression onto a hilly terrain mantled with UAF and slope breccias, (b) the succession hosts a section with a breccia bed that is provisionally interpreted as a tsunamite, and (c) contains beds that record coseismic deformation. Aside of first documenting this type of basal interval of the Gosau Group, it is the intention of this contribution to alert for similar evidence in other areas.

Regional

From Late Permian to Triassic times, the area of the Eastern Alps (Fig. 1a) was located on a south-facing passive margin (e.g., Oberhauser 1995; Le Breton et al. 2021). Because of a peneplained hinterland and a position in the subtropical high-pressure belt, the Triassic succession is dominated by thick carbonate platforms. During the Early Jurassic, the Eastern-Alpine (or Austroalpine) domain was subject to marked deepening associated with oblique rifting (Bosellini and Winterer 1975; Weissert and Bernoulli 1985; Channell et al. 1990). Deep-water conditions prevailed until the latest Jurassic, when oblique tectonic convergence related to Alpine orogenesis began. During the Early Cretaceous, stacking of W- to NW- vergent fold- and thrust nappes was accompanied by synorogenic deposition (Decker et al. 1987; Ratschbacher 1987; Eynatten and Gaupp 1999). During the late Albian to Cenomanian, large parts of the orogenic wedge were uplifted and exposed, and a regional truncation surface was incised into the folded and thrusted rocks (Gaupp 1983; Wagreich and Faupl 1994).

From the middle Turonian onward, subsidence resumed, and the orogenic wedge was onlapped and overstepped by the succession of the Gosau Group (Turonian to Ypresian). At many locations, Gosau deposition starts with alluvial-fan successions up to a few hundreds of metres in thickness that contain a dark red, terra-rossa like matrix produced by weathering in a warm-humid climate (Wagreich and Faupl 1994). Where alluvial-fan deposits are absent, the basal intervals that overlie the older substrate may comprise other, terrestrial to paralic deposits, but basal sections are only rarely well-exposed. Based on a major unconformity within the succession (see Supplementary Fig. S1), two subgroups are distinguished: (1) The Lower Gosau Subgroup (LGS), comprising a wide spectrum of terrestrial to neritic deposits, including the mentioned alluvial-fan successions, and (2) the Upper Gosau Subgroup (UGS) that – aside of a basal transgressive interval – records a deepening into bathyal to abyssal environments (Faupl et al. 1987; Wagreich and Faupl 1994; Koukal et al. 2022). The transgressive interval along the base of the UGS typically records pebbly-bouldery to rocky shores, ahead of narrow and steep shelves. The basal section described herein pertains to the LGS.

Because of the oblique-convergent setting and resulting strain partitioning, the accumulation of the wedge-top succession was ascribed to different local tectonic settings, such as extensional to transtensional basins (e.g.,Wagreich 1995; Wagreich and Decker 2001; Neubauer and Genser 2018), basins formed along transfer faults (Eisbacher and Brandner 1996) and piggy-back basins (Ortner 2001; Ortner et al. 2016). Due to tectonic indentation of the Southern Alpine crustal block into the nappe stack of the Eastern Alps, during the Neogene, the Eastern Alpine edifice became dismembered along wrench faults oblique to parallel to the orogen trend (Ratschbacher et al. 1991; Linzer et al. 1995; 1997). The eastern part of the Inn valley (Fig. 1a) follows a sinistral wrench belt that became active during the Neogene (Linzer et al. 1997; Ortner and Stingl 2001). The fold-and-thrust belt north of the Inn valley comprises: (a) thick packages of Triassic shallow-water carbonate rocks and, subordinately, (b) Jurassic to Lower Cretaceous deep-water limestones, cherts and marls, unconformably overlain by (c) the deposits of the Gosau Group.

Study area

The study area is part of an outcrop ~ 9 x 8 km in size north of the Inn valley of an upper Turonian to lower Santonian succession (Supplementary Fig. S1) (cf., Herm et al. 1979; Tröger and Summesberger 1994; Sanders 1996, 1997, 1998; Summesberger and Kennedy 1996; Wagreich et al. 2009). The Gosau deposits became preserved by downthrow along normal to oblique-slip faults associated with releasing bends of wrench faults that shortly predated or accompanied the individuation of the Neogene Inn-valley strike-slip fault zone (cf., Sanders and Gruber 2023, their Fig. 2). In the study area, the Gosau Group overlies a truncated substrate of folded and thrusted Triassic platform dolostones (Hauptdolomit unit) (Figs. 1b, c, 2). The studied interval comprises the basal portion of an ?upper Turonian to middle Coniacian mixed siliciclastic-carbonate succession ~ 200 m in thickness (Fig. 3). Whereas the lower part of the succession mainly includes paralic arenites with intercalated shell beds, the upper part comprises inner shelf deposits with coral-rudist buildups (Fig. 1c and Supplementary Fig. S1) (Herm et al. 1979; Sanders and Baron-Szabo 1997; Sanders 1998, for details). Up to 2010, the basal part of this succession was poorly exposed; scattered outcrops showed that the succession started with dololithic breccias and stream conglomerates, whereas the better-exposed section started only a few metres stratigraphically higher up (Sanders and Baron-Szabo 1997). As mentioned, in 2011, the area directly north of Kreuthalm was cleared and new forest roads were excavated, exposing the basal part of the succession described herein (Figs. 2, 3).

The studied interval was mapped in the field on a scale of 1:1000 in 1-m resolved laserscan topography provided free in the net by the federal country of Tyrol (https://maps.tirol.gv.at/). Orientations of bedding or other planar features are given in the dip azimuth/dip notation. Thirty cut and polished rock samples and 22 thin sections 4 x 6 cm in area supported documentation of facies. Thin sections were inspected with an Olympus SZX10 binocular with nicols and a device for optional dark-field illumination that may better highlight crystal shapes and contrast between clasts and matrix. The designation of grain sizes follows the Wentworth (1922) scale. Bed thickness and clast axes are designated according to Collinson and Mountney (2019).

In the following, an interval of carbonate rock is dubbed as 'intact' if primary depositional features such as loferites or stromatolites are preserved over at least a few decimetres of exposure without being offset by faulting. Intactness, thus, does not include joints, which per definition are fractures without offset (Ramsay and Huber, 1987). Cataclasites are designated according to the Sibson (1977) classification. Fragmentites may consist of subclasts that display a variable degree of fitting: depending on the degree the subclasts are translated/rotated relative to each other, three types of subclast fabrics are distinguished: (1) crackle fabric, wherein a clast is crossed by fractures, but the subclasts are not or nearly not removed from each other; (2) jigsaw fabric, wherein the subclasts are slightly translated from each other; and (3) mosaic fabric, wherein the subclasts (in particular marginal ones) are significantly translated and rotated (see Sanders et al. 2022, for summary). If not addressed specifically, in the following, these three subclast fabrics are collectively designated as 'clasts fractured in situ'. Shanmugam (2012) had pointed out that the deposit category tsunamite is invalid because no process of transport and deposition is unique to tsunami-induced flow. Nevertheless, to address an event deposit that originated in association with a tsunami, hereunder the term will be used in ampersands. In the following, the geological situation in the study area is described in ascending order from the bedrock to the topmost deposits of interest here.

Hauptdolomit rock substrate and proto-/cataclasites

The dolostone succession defined as 'intact rock' is densely jointed throughout (point 1 in Fig. 2, Fig. 4a). Locally, the intact Hauptdolomit is topped by an interval up to ~ 10 m in thickness of fragmentites with a protocataclasite to cataclasite fabric (points 2 in Fig. 2, Fig. 4b). Whereas some of these (proto-)cataclasites clearly are associated with younger faults (small white arrows in Fig. 2), the mapped distribution of the (proto-)cataclasites (labeled 2 in Fig. 2) clearly shows that they comprise a 'sheet-like' lithosome above steeper-dipping, intact Hauptdolomit underneath. Aside of a faint, very thick subhorizontal stratification, these sheet-like fragmentites do not display identifiable fabrics. Thin sections display variable degrees of cataclasis, ranging from protocataclasites that may contain small karstic pockets (Fig. 4c, d) to cataclasites with a matrix of lithified gouge (Fig. 4e, f).

Slope breccias

At several locations (e.g., points 3 in Fig. 2), intact Hauptdolomit or (proto-)cataclasites are overlain by: (a) clast-supported breccias that display mixing and rotation of subangular to subrounded clasts relative to each other (Fig. 5a–c), and that (b) may display clear-cut subparallel stratification (Fig. 5d). In thin section, a few of the clasts display a fragmentation into crackle to jigsaw fabric (Fig. 5c, e). The matrix of these breccias is a poorly-sorted doloclastic siltite to arenite (Fig. 5f).

Stream conglomerates

Intact Hauptdolomit or, in one case, an interval of clast-supported doloclastic breccia, locally is overlain by clast-supported, fine pebbly to small cobbly conglomerates (points 4 in Fig. 2, Fig. 6a, b) that consist of very well-rounded clasts of typically medium- to coarse-crystalline dolostones (Fig. 6c, d). In the conglomerates, clasts display concavo-convex, planar clast contacts but, despite mutual impingement, in most cases are intact (not fractured) (Fig. 6c, d). Only locally, conglomerate clasts are disintegrated into crackle to jigsaw subclast fabrics, with the resulting pore space between the subclasts filled with the matrix the conglomerates otherwise contain (Fig. 6e, f).

Event bed

At location 5 (Fig. 2), the new roadcut exposed a section that comprises three intervals (Fig. 7): interval 1 of poorly-sorted bioclastic limestone, sharply overlain by interval 2, a very poorly-sorted dololithic rudstone with a few large bioclasts (rudists, corals), in turn sharply overlain by interval 3, a poorly-sorted bioclastic limestone with gastropods (e.g., actaeonellids, Cassiope) (Figs. 7, 8a, Table 1).

Table 1

Bioclasts identified in interval 2 of the section shown in Fig. 7
Bioclast type	Remarks, figure reference
Fragments of large gastropods	Common
'Porostromates' (possible cyanoids)	Common; Fig. 11a
Leiolithic microbialites	Stoutly branched structures of micrite; Figure 11d
Fragments of rudists	Common
Rhynchonellid shells (disarticulated)	Common; Fig. 11a
High-spired microgastropods	Common
Fragments of echinoderms	Uncommon; Fig. 11a
Fragments of non-rudist bivalves	Uncommon
Fragments of serpulid tubes	Uncommon; Fig. 11c
Ostracods in articulated preservation	Uncommon
Coralline algae	Uncommon; branched morphs
Smaller benthic foraminifera (mainly Miliolina)	Uncommon
Small fragments of (angiosperm) wood	Uncommon
Fragments of inoceramid shells (thick prismatic outer shell layer)	Rare; Fig. 11b
Remnant of calcareous green alga?	Single find in pseudosparite, poorly preserved

Interval 1, ~ 15 cm in thickness, is a bioturbated, very poorly-sorted bioclastic floatstone with microbialite-coated grains and with fragments of colonial corals and rudists (Fig. 8b, c). In addition, a few angular dolostone clasts up to medium-pebble size float within the matrix (Fig. 8d, e). In contrast to the smooth surfaces of the clasts in the conglomerate (cf., Fig. 6c), the surfaces of dolostone clasts in interval 1 are rugged (Fig. 8e). The matrix of the floatstone is a poorly-sorted bioclastic wackestone with small coalified plant fragments and with small, more-or-less blackened bioclasts, and with a few grains of siliciclastic sand (Fig. 8b–d).

In the field, interval 1 appears sharply overlain by interval 2a (cf., Figs. 7, 8a). Rock samples, however, reveal that the basal part of interval 2a is a level a few centimetres thick of dololithic float- to rudstone with a matrix of poorly-sorted lithic arenite, whereas the overlying main part of interval 2a consists only of rudstone of angular dolostone fragments (Figs. 8f, 9a and Supplementary Fig. S2). The matrix of the basal part is not different from the remainder of interval 2a: a poorly-sorted doloclastic siltite to arenite with a few grains of quartz and quartzite rock fragments (Fig. 9b, c). The clast fabric of interval 2a is densely packed, locally with clast contacts overprinted by mechanical compaction and pressure solution (Fig. 9d). A further feature is that many lithoclasts are disintegrated into crackle to mosaic subclast fabrics (Fig. 9d–f), despite the presence of the arenite matrix (Fig. 10a).

In interval 2b, lithoclasts are less densely packed, giving rise to rudstone to local floatstone fabric, and a few large clasts of rudists and corals are seen in the field.

A few dolostone clasts show rounded and deeply embayed outlines (Fig. 10b) whereas the surface of these clasts, but also the original 'outer' surfaces of clasts that became disintegrated into subclasts, is rugged because of projecting dolomite crystals (Fig. 10c). Over large parts of interval 2, the matrix displays a fabric of equigranular coarse calcite spar with highly complicated, 'amoeboid' boundaries between individual crystals, and with components apparently floating within the spar (Fig. 10d–f). The matrix of interval 2, in particular of interval 2b, contains a wide spectrum of bioclasts, such as from serpulids, inoceramids, brachiopods, cyanoids, and coralline algae (Fig. 11a–d; Table 1). Bioclasts are more common in interval 2b, in particular larger clasts of radiolitid rudists or colonial corals (Fig. 7). The features of interval 2 are summarized in Table 2.

Table 2

Elements of interpretation of interval 2 of the section shown in Fig. 7.
Context and bed characteristics
(1) Interval 2 under/overlain by limestones of protected, shallow subtidal environments (2) Interval 2: 40 cm thick (3) Interval 2: Clast-supported dolo-lithic breccia of fine pebbles to cobbles (4) Lithoclasts derived from local rock substrate (Triassic dolostones) (5) Interval 2 shows sharp base and sharp top to under/overlying deposits (6) Interval 2 is not graded
Internal features of bed
(7) Basal level ~ 5 cm in thickness of interval 2 inversely graded (8) Interval 2 shows subtle bipartite subdivision into: (a) a lower part of dolostone clasts in a sparse matrix of lithic arenite, and (b) an upper part of dolostone clasts in a matrix of hybrid arenite with scattered larger bioclasts (9) Breccia clasts show a 'rugged' surface as typical of chemical weathering of coarse-crystalline dolostones (10) Matrix is a hybrid arenite of: carbonate-lithic grains, (b) diverse siliciclastic grains (see text), (c) bioclasts of normal-marine to, perhaps, restricted (schizohaline) marginal-marine settings (11) Matrix of interval 2 contrasts with lime-muddy matrices of under/overlying bed (12) Bioclasts in matrix are fragmented, but do not show micrite rims or (micro)borings

The topmost interval 3 of the section contains a few fully-preserved actaeonellids and the cerithiacean Cassiope. This interval consists of very poorly-sorted, bioturbated bioclastic floatstones with a matrix of bioclastic wackestone (Fig. 11e). This limestone displays a diversified spectrum of bioclasts of marine shallow-water habitats, some of them coated by cyanolithic crusts. Interval 3 also contains dolostone clasts that, either, may display a smooth surface and a subangular to subrounded shape, whereas others show a very angular shape and a rugged surface (Fig. 11f). In addition, pieces of coalified angiosperm wood are present.

Cairns of event beds

In areas 6 and 7 (see Fig. 2), cairns of cobble to small boulder size of a specific facies type are littered; it is assumed that these cairns are part of the transgressive succession at site, yet unfortunately not exposed. The facies of the cairns comprises clast- to matrix-supported dololithic breccio-conglomerates with a matrix of hybrid arenite with a few macrofossils, such as actaeonellids (Fig. 12). Aside of an indistinct preferred orientation of the [a,b]-planes of lithoclasts subparallel to what may be inferred to be depositional stratification, and aside of a few indistinct imbricated clast fabrics (Fig. 12a), no primary sedimentary structures were identified. Many lithoclasts display planar or convexo-concave contacts (Fig. 12b–d), and are crossed by fractures or are more-or-less crushed. Open fractures between subclast fragments are filled with the matrix arenite (Fig. 12b–d). Locally, the fractures across lithoclasts are represented by light-weathered planes that cut through both the clast and the adjacent matrix arenite (Fig. 12e). Cairns of a similar facies were found at location 7, but in these, the evidence for fracturation into subclasts along mutual clast contacts was absent. Nevertheless, also these cairns supported the interpretation of this facies type (see below).

Rock substrate and proto/cataclasites

In the NCA, with their long-ranging deformation (see Setting), the density of jointing seen in the 'intact' Hauptdolomit (point 1 in Fig. 2, Fig. 4a) is typical. Dolomite deforms in a ductile manner only above 450–500°C (Dietrich 1983); in the NCA, such a temperature was not attained during folding and thrusting (cf., Kralik et al. 1994). The proto/cataclasites that overlie the intact Hauptdolomit along a gently dipping, planar contact (point 2 in Fig. 2, Fig. 4b) may represent damage zones of faults, such as a thrust ramp or a low-angle normal fault. Within the interval of proto/cataclasites, no fault core was seen; together with the observation that the interval is underlain by intact Hauptdolomit, this suggests that the proto/cataclasites represent an erosional vestige of a fault footwall. A few kilometres south of the study area, along the southern margin of the NCA, low-angle detachment faults of Jurassic to Cretaceous age are documented (Ortner and Reiter 1999). The observation that the proto/cataclasites are onlapped and overstepped by Gosau deposits that were not subject to wholesale cataclasis underscores that the putative hangingwall (of whatever fault type inferred) was eroded before onlap.

In thin section, the proto/cataclasites show a size continuum of rock fragments generated by fracture and attrition, down to a matrix of lithified doloclastic gouge (Fig. 4c–f); fragments up from approximately fine pebble size display jigsaw to mosaic subclast fabrics. In total, from field to thin section, the proto/cataclasites are well-comparable to damage zones (not fault cores) in carbonate rocks (cf., Billi et al. 2003; Billi 2005; Hausegger et al. 2010; Ortner et al. 2018). The low dip of the proto/cataclasite interval (cf., Fig. 2) suggests that it is a vestige of a damage zone related to folding and thrusting (e.g., Tarasewicz et al. 2005) or to extensional faulting (Agosta and Aydin 2006; Demurtas et al. 2016; Ortner et al. 2022). Alternatively, the interval of proto/cataclasites may be interpreted as a rhegolith that originated from the densely-jointed 'intact' Hauptdolomit by – up-section increasing – translation/rotation of joint-delimited clasts (cf., Fig. 4a) relative to each other. In this case, the clasts should be embedded in a secondary matrix infiltrated from the exposed area above. Indeed, small 'pockets' of red-coloured internal sediment – presumably related to karstification during subaerial exposure – are locally present in the proto/cataclasite (Fig. 4d), but the overwhelming part of the matrix is of cataclastic origin (Fig. 4e, f). This supports the interpretation of the proto/cataclasites as a damage zone related to a pre-Gosau fault.

Slope breccias

These breccias differ from the cataclasites in that: (a) they consist of or contain a sizeable proportion of clasts of subangular to subrounded shape, (b) clasts are so much translated/rotated relative to each other that clast boundaries cannot be matched, (c) despite very poor sorting, there is a distinct jump in grain size between the breccia clasts and the matrix, and (d) these breccias are stratified (Fig. 5b–d). Nevertheless, both the larger clasts and the clastic matrix are derived from the local dolostone substrate (Fig. 5c, f), and no extraclasts derived from other sources was identified.

These breccias are interpreted as hillslope mantles mainly formed by slow transport processes such as downslope creep. If the breccias had formed, mainly at least, by relatively dilute and rapidly-propagating cohesive debris flows, typical features of such flows such as inverse grading and vertical size sorting of clasts, a-axis imbrication of larger clasts, and intercalated levels of fine-grained material deposited from overland flow in between debris-flow events should be present (cf., Sanders et al. 2009; Ventra et al. 2013); all this was not observed. The significance of localized presence of crackle– to jigsaw subclast fabrics (Fig. 5c, e) in these breccias is dealt with farther below.

Stream conglomerates

In transgressive situations such as the one considered herein, conglomerates may accumulate along a beachface or from streams. Beachface conglomerates in the Gosau Group commonly are associated with fan deltas and contain a matrix of mixed carbonate-lithic/siliciclastic arenite and, rarely, marine bioclasts (Wagreich 1989; Sanders 1997, 1998, 2001). The conglomerates of interest herein, in contrast, consist only of dolostone clasts within a sparse doloclastic matrix. Therefore, an interpretation in terms of stream conglomerates is preferred. Unfortunately, the outcrops are not of sufficient extent to support an unequivocal distinction between beachface or stream conglomerates. In the studied conglomerates, the good rounding and high sphericity of clasts (Fig. 6a–d) suggests a transport of a few kilometres at least. Better preserved successions of unequivocal stream deposits of similar character are exposed at the base of the Gosau Group a few kilometres farther east of the study area (Sanders and Gruber 2023), and support the possibility that such conglomerates are locally present. The question of localized in-situ clast fracture in the conglomerates (Fig. 6e, f) is treated farther below.

Section with event bed

Intervals 1 and 3: In interval 1 (Fig. 7), the bioclasts of corals and rudists, small wood fragments, and angular clasts of dolostone come from different environments: fragments of wood and dolostone clasts from areas onshore, and corals and rudists from shallow-marine settings. The lime-muddy bioturbated matrix (Fig. 8b–d) records a setting of overall low energy, yet the marine- and land-derived components suggests mixing during episodic high-energy events (cf., Mount 1984). To carry land-derived components into a shallow-marine setting requires offshore-directed transport such as upon coastal submergence by storms or tsunami (see below for discussion). The angular shape and the 'rugged' outline of the dolostone clasts (Fig. 8d, e) suggests that these clasts had lodged within soil or in other sediments (scree?) before reworking and embedding in the matrix of interval 1. Relative to the record in the overlying interval 2, the postulated high-energy events that led to component mixing were of limited impact. Notably, all of the characteristics of interval 3 are closely similar to those of interval 1, and record an overall quiet shallow subtidal setting ('bay', 'lagoon') punctuated by high-energy events of limited outreach with respect to component mixing.

Interval 2: As described, the sharp-based interval 2a (Fig. 7) is an orthobreccia of fine-pebbly to small-cobbly dolostone clasts in a sparse matrix of lithic arenite (Figs. 9b–f, 10a). The shapes – from angular to embayed to well-rounded (Fig. 10b) – and surfaces from smooth to rugged, of the dolostone clasts, suggest that they came from different environments: Well-rounded, smooth clasts may from a stream bed and/or a beachface; the prevalent fraction of angular clasts with rugged and embayed surfaces, in turn, might have rested on land in soil, scree slopes, or debris flows. No lithoclast infested by marine macroborers was observed. With the potential exception of serpulids and cyanoids that may tolerate schizohaline/brackish conditions, the bioclast spectrum is dominated by taxa indicative of normal-marine environments (Table 1). The portions of interval 2 that, in their matrix, display an equigranular coarse calcite spar with 'amoeboid' crystal boundaries, and with components apparently floating within, represent pseudosparite (Fig. 10d–f). The Hjulstrøm curve suggests that to transport small cobbles ~ 10 cm in size even if only by traction requires a flow ~ 2–3 m/s in velocity, which provides a crude impression of minimum current velocities required to keep clasts in motion. Because at least most of the lithoclasts of interval 2 were mobilized from terrestrial areas, this implies that coastal submergence was followed by an offshore-directed flow strong enough to transport small cobbles.

Deposition of interval 2 is envisioned as follows: During an event of exceptional coastal flooding, a wide onshore strip was submerged by a surge with shallow-marine bioclasts. During or close to the end of submergence, seaward backflow transported clasts up to at least cobble size from the submerged onshore area to the sea. At the site of observation (Fig. 7), this backflow deposited part of its coarse load. The lack of organization with respect to clast size across the bed, and the matrix of winnowed arenite all suggest that deposition was rapid, such that the clasts collided with each other in a traction carpet (e.g., Todd 1989; Sohn et al. 1997, Sohn 2000; Moore et al. 2011). The subtle bipartite division of interval 2 may result from a twofold backflow, or from a transformation of a single backflow by deposition mainly of lithoclasts (cf., Sakuna et al. 2012).

Most features considered typical of storm-induced deposition were equally plead to support an origin from tsunami (cf., Bahlburg et al. 2010; Shanmugam 2012; Paris et al. 2018). Along active margins, seismic faulting and submarine mass-wasting can generate tsunami runup > 10 m in amplitude (e.g., Borrero et al. 2001; Bohannon and Gardner 2004; Locat et al. 2004; Spiske and Bahlburg 2011; Spiske et al. 2014; Ryan et al. 2015; Takashimizu et al. 2020). Tsunami typically run up with 5 to ~ 20m/s (e.g., Jaffe and Gelfenbaum 2007; Yeh and Li 2008; Spiske and Bahlburg 2011), sufficient to mobilize boulders. Tsunami impact typically comprises several runup/backwash pulses, but the first pulse usually is highest. Offshore 'tsunamites' thus may comprise a single or several layers, respectively (e.g., Massari et al. 2009; Sakuna et al. 2012). For geological preservation, the highly erosive tsunami backwash (up to > 10 m/s) of hyperconcentrated flows or grain flows is more relevant than uprush (Fujiwara et al. 2000; Le Roux and Vargas 2005; Dawson and Stewart 2007; Yeh and Li 2008; MacInnes et al. 2009; Paris et al. 2010; Spiske and Bahlburg 2011; Varela et al. 2011; Feldens et al. 2012; Spiske et al. 2014; Gusman et al. 2018; Slootman et al. 2018). In the present example, flooding of a hilly coastscape was followed by a backflow that carried the mix of land-derived lithoclasts and marine bioclasts into a gently-dipping shallow subtidal environment (cf., Sakuna-Schwartz et al. 2015). The thickness of interval 2, and its composition of clasts up to cobble size, are well-comparable to historical offshore tsunamites (cf., Sakuna et al. 2012, their Table 2). The sharp contact between interval 2 and 3 perhaps results from partial reworking of the original top by storms or swell (cf., Weiss and Bahlburg 2006).

Storm-induced flooding also comprises flood and ebb surges with amplitudes similar to tsunami (e.g., Coch 1994, Borrero et al. 2004; Weisberg and Zheng 2006; Robertson et al. 2007). Pebbles to cobbles from land may initially have been mobilized during storm flood (velocity up to a few m/s; cf., Robertson et al. 2007), then carried offshore by ebb surge. In contrast to a wave period of ~ 10 min to > 1 h of tsunami (Weiss and Bahlburg 2006), storm surges wane over 5 to 10 h, implying a relatively gentle ebb flow (e.g., Weisberg and Zheng 2006). Storm ebb surges may cut channels in the shoreface, and can transport sediment out to a few hundreds of metres off shore into waters a few metres deep (Coch 1994). 'Tsunamites' typically comprise 1 to 4 layers, but the same was also observed in historical storm layers (Phantuwongraj and Choowong 2012). Similarly, mixing of fossils from different environments and angular bioclasts are considered typical of bay/lagoonal 'tsunamites', but the same can also be achieved by storm surges (Fujiwara et al. 2000; Dawson and Stewart 2007; Varela et al. 2011). Field data on the bottom velocity of storm-induced offshore flows are unavailable; most data are from measurements 1 m or more above sea bed, where flow might be swifter than directly above ground. On an open shelf, offshore storm flows and surge ebbs may attain rare peak velocities of 1-1.5 m/s at 3 m above sea bed (Morton 1981). In macrotidal estuaries on wide gently sloping shelves, spring tides superposed with typhoons may induce ebb surges up to > 2 m/s at sea surface (Zhang et al. 2012). Numerical modeling of storm surge ebb on open shelves indicates velocities of up to 1.6 m/s sustained over less than an hour (Suter et al. 1982). In any case, even if transferred 1:1 to the sediment surface, these flow velocities are clearly on the lower verge required for transport cobble-sized sediment.

Alternatively, interval 2 might be interpreted as a debris flow that originated on a hillslope on land (without flooding) and intruded into the shallow marine area. This seems precluded because: (1) The arenite matrix with marine bioclasts is inconsistent with the unfossiliferous terra rossa-like matrix as it is widespread in terrestrial deposits of the Gosau Group; (2) If it is postulated that the debris flow diluted by uptake of sea water and mixed in marine-derived material (sand, bioclasts) to the extent that its original matrix was completely exchanged, it should have passed a fully turbulent state including suspension while propagating under water, i.e., it was no longer a debris flow. It further seems doubtful whether the low gradient of the shallow marine area the flow intruded into provided enough slope energy to allow for such a process. As discussed, the debris-flow like features of interval 2 can be explained by rapid deposition from a grain flow within a traction carpet along the base of a flow of unknown total depth. In summary, the event bed is tentatively interpreted in terms of a 'tsunamite' because it seems most straightforward to derive from processes associated with tsunami runup and backflow.

Cairns of breccio-conglomerates

The dolostone clasts in the cairns of breccio-conglomerates (Fig. 12) may stem from the debouch of small streams or fan deltas, or from a transgressive beachface. The clast- to matrix-supported fabric, and the poorly-defined clast fabric aside of a crude parallel alignment of clast [a,b] planes suggest rapid deposition. If the breccio-conglomerates formed by tsunami backflow from flooded coastal areas, a much more diversified mixture of bioclasts and lithoclasts may be expected. Redistribution of beachface to upper shoreface sediments into final deposits typically takes place during beach reformation upon waning storms or swells (Sanders 2000). In summary, these deposits are preferably interpreted as shore-zone deposits accumulated during waning of storms or swell.

Reconstruction of depositional setting

In attempting an environmental reconstruction, the stratigraphic contacts in the mapped area are reiterated (Fig. 13). The most complete synthetic section (column A, Fig. 13) comprises all lithologies up to a first interval of shoreface conglomerate. Choosing the boundary between intact Hauptdolomit and overlying rocks as a correlation datum, it is seen that the proto/cataclasites are not everywhere present below the basal Gosau deposits (columns B and C, Fig. 13); yet, slope breccias also are not present everywhere in contact with the Hauptdolomit, and so are the stream deposits (Fig. 13). These relations may be explained by: (a) partial subaerial erosion of the protocataclasite interval, leading to (b) redeposition of protocataclasite fragments into slope breccias, succeeded by (c) incision of a stream that, upon (d) channel backfilling due to base-level rise onlapped and overstepped the slope mantles (Fig. 13). Presumably later, and stratigraphically higher up, a very shallow marine-subtidal setting ('lagoon') established that contains the section with the interpreted 'tsunamite' (Fig. 7). Still higher up, shoreface conglomerates and marine hybrid arenites accumulated (location 4, Fig. 2; Figs. 13a, 14; cf., Sanders 1998).

Significance of in situ clast fragmentation

Letting aside the proto/cataclasites, which per se are rocks formed by brittle shear, in all of the described deposits there is evidence for in situ clast disintegration (= while embedded in sediment) ranging from crackle to mosaic fabrics, with fracture pores filled with the same primary matrix as seen in the rest of the deposit (Figs. 5c, e, 6e, f, 9e, f, 12). If mere overburden had caused clast fracture, this should be seen in all clast-supported fabrics. Instead, also in conglomerates, overburden induced pressure solution along convexo-concave contacts while the majority of clasts remained unfractured (Fig. 6c, d). The observed clast/matrix fabrics and the deformation bands that cross-cut clasts (Fig. 12), are inconsistent with fracture merely by sediment load. A hallmark of deposits of rapid, catastrophic mass-wasting (CMW = rockslides, rock avalanches; Evans et al. 2006) is pervasive in-situ clast fracture by dynamic disintegration (e.g., Davies and MacSaveney 1999, 2002, 2009; Dufresne et al. 2016; Schilirò et al. 2019; Sanders et al. 2022; Sanders and Gruber 2023). The matrix of CMW deposits is a cataclastic gouge generated by clast fracture and attrition such that a 'self-similar' grain-size distribution results (cf., Perinotto et al. 2015; Dufresne et al. 2016). In the studied Gosau succession, thus, the clasts fractured in situ most probably were produced during seismic events. A characteristic of coseismically fractured coarse-clastic deposits is that fracturation is confined to ensembles of clasts and/or to subvertical belts of clasts, whereas the clasts in the adjacent sediment are intact (cf., Meyer et al. 2006; Tuitz et al. 2010; Kübler et al. 2017; Sanders et al. 2018; Tokarski and Strzelecki 2020). This feature fits with the patchy distribution of clasts fractured in situ observed in the studied Gosau deposits. Furthermore, non-stylolithic planar contacts between clasts (Fig. 11), be they fractured or not, are characteristic of coarse-clastic sediments compacted by coseismic rocking (Sanders et al. 2018). Soft-sediment deformation structures that presumably are produced by early post-depositional coseismic displacement are present also higher up in the succession at site (see Fig. 3 and Supplementary Fig. S3). Because the arenitic sediment affected by coseismic deformation was not fully lithified, these structures lack features of brittle deformation such as cataclasis, slickensides, and Riedel shears (Supplementary Fig. S3).

A few kilometres south of the study area, along the southern margin of the NCA, Ortner and Reiter (1999) had identified Jurassic-Cretaceous, top-SE low-angle detachments that cut from Triassic carbonate rocks down into older (anchi-) metamorphic rocks. Such detachments, and associated secondary faults, produce accomodation space for wedge-top basins and may explain the intervals of cataclasites directly below the base of Gosau successions. Similar cataclasites are locally present along the base of the Gosau at other locations, but so far were not mapped or otherwise studied in detail. Coseismic deformation structures in successions of wedge-top basins seem hardly surprising, yet, in the Gosau Group, it seems that such features to date were nearly totally overgone (Sanders 2001). Together, (i) fragmentites in the rock substrate directly below, (ii) coseismic deformation structures within Gosau deposits, and (iii) paralic event beds that result from coastal inundation may be viewed as a 'trinity' related to the synorogenic tectonic setting.

(1) Along the base of an Upper Cretaceous synorogenic wedge-top succession (Gosau Group, Eastern Alps), above truncated Triassic dolostones, an assemblage was observed that includes: (a) proto-/cataclasites, (b) slope breccias, (c) stream conglomerates, and (d) low-energy 'lagoonal' limestones with an intercalated event bed. The proto-/cataclasites represent a vestige of a damage zone related to synorogenic faulting. Slope breccias consist of subangular to subrounded clasts from the local substrate, display stratification, and contain a matrix of doloarenite to siltite. The stream conglomerates are devoid of matrix, and accumulated from a small flume incised down to the Triassic dolostones. The 'lagoonal' limestones are bio-lithoclastic floatstones with angular clasts of corals, rudists, gastropods and fragments of wood. Angular dolostone pebbles with a corrugated surface were derived from of onshore areas during events of moderate coastal flooding.

(2) Together, the facies and their arrangement in space suggest a transgressed landscape that was hilly (local relief of a few meters to a few tens of meters), forested, and drained by a small stream, or streams. Upon transgression, a low-energy marginal marine ('lagoonal') environment established that was punctuated by high-energy events: coastal flooding during storms or low-amplitude tsunami flooding had carried lithoclasts to the lagoonal limestone setting.

(3) Within lagoonal limestones, a clast-supported bed ~ 40 cm thick is sharply intercalated that consists of angular clasts of Triassic dolostone of pebble to cobble size with a corrugated surface. The matrix is a hybrid arenite with bioclasts of rudists, brachiopods, red algae, corals, inoceramids and other bivalves. The bed records an event of exceptional coastal flooding and is tentatively interpreted as a deposit of tsunami backflow, whereas an origin from storm surge cannot be excluded with complete certainty.

(4) Fabrics that record fracture of lithoclasts while embedded in soft sediment (crackle to mosaic subclast fabrics) with primary matrix between subclasts are common. These fabrics, not pervasive but confined to patches, record coseismic fracture. With respect to clasts fractured in situ, a typical patchiness of coseismic deformation and the fact that it affects sediments of different origins, terrestrial to marine, helps in distinction from erosional vestiges of deposits of catastrophic mass-wastings (rock avalanches, rockslides). Soft-sediment deformation of probable coseismic origin was observed also higher up in the transgressive succession at site.

(5) The study area provides a rarely exposed example for the most basal part of an onlapping synorogenic succession of terrestrial to marginal-marine environments. Damage zones of proto-/cataclasites directly below onlapping deposits hint on deformation before synorogenic deposition. The distinction of event beds related to coastal flooding by storms or tsunami remains problematic, but potential records thereof should be searched for. Coseismic deformation expectably is widespread in particular in the basal and lower part of the Gosau Group, but to date was largely overgone.

Supplementary Information

The online version contains supplementary materials available at . . .

Acknowledgements

Discussions and collaboration with Alfred Gruber, Geosphere Austria (Vienna) and Wolfgang Thöny, OMV (Vienna) on the manyfold varieties of the basal parts of transgressive successions drove the motivation to present the results in this paper.

Funding

The author did not receive funding of any kind for the present study.

Data availability

Data relevant for this study are contained in this article and in the Online Supplementaries. Further data are available on request to the author.

Conflict of interest

The author has no competing interests to declare that would interfere with the content of this article.

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Editorial decision: Major Revision
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Onlap of Upper Cretaceous synorogenic wedge-top deposits (Eastern Alps): Fragmentites, event beds, and coseismic deformation.

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Version 1

Abstract

Figures

Introduction

Setting

Regional

Study area

Methods and definitions

Description

Hauptdolomit rock substrate and proto-/cataclasites

Slope breccias

Stream conglomerates

Event bed

Cairns of event beds

Interpretation and discussion

Rock substrate and proto/cataclasites

Slope breccias

Stream conglomerates

Section with event bed

Cairns of breccio-conglomerates

Reconstruction of depositional setting

Significance of in situ clast fragmentation

Conclusions

Declarations

References

Supplementary Files

Status:

Version 1