Katarina Gobo

The sedimentary infill of peripheral foreland basins records an intricate relationship between the development of the thrust wedge and related isostatic adjustments, eustasy, and sedimentation processes, posing challenges to the classical source-to-sink concept. The evolution of such basins commences with an “underfilled” flysch stage in the foredeep depozone and passes to a filled and finally an “overfilled” molasse stage in the orogen-proximal zone (Sinclair, 1997). On basin-scale, progressively younger sediments exhibit a regressive character, with the molasse commonly being deposited in alluvial environments.This study from the North Dalmatian foreland basin in Croatia documents a peculiar gravel association in the alluvial conglomerates of the foreland molasse. These deposits are the youngest unit of the Promina Beds – a calciclastic succession that was deposited in the wedge-top depozone in shallow-marine, marginal marine, and terrestrial environments from the middle Eocene to the late Oligocene (Mrinjek et al., 2012). The alluvial conglomerates were deposited on Oligocene alluvial fans or in proximal braided-river settings (Mrinjek et al., 2012). Besides limestone clasts derived from Cretaceous bedrock, these conglomerates comprise well-rounded calciclastic cobbles and boulders. These clasts are fine-grained calcarenites, whose carbonate content ranges from 61 to 99%. Their macroscopic features resemble sandstones of Eocene flysch that crop out 16 km to the southwest, but samples from these two units show significant differences in microfossil content and heavy mineral associations. Large benthic foraminifera and sporadic planktic forms are found in the Eocene flysch, which was deposited in the prodelta zone of a river-fed delta (Babić & Zupanič, 2008). Contrarily, the calciclastic gravel clasts of the alluvial unit bear more planktic than benthic foraminifera, suggesting original deposition in a deeper and more distal marine environment. The clasts are rich in muscovite (32%), opaque heavy minerals (24%), and biotite (19%), with subordinate transparent heavy minerals (12%). Flysch samples show a coequal abundance of opaque (39%) and transparent heavy minerals (40%). Among the latter, garnet is the most abundant.These preliminary results suggest that the calciclastic gravel clasts of the alluvial unit most likely derive from a foredeep setting older than the Eocene flysch, that was probably located in the hinterland of the present-day thrust-wedge. The heavy mineral assemblage suggests provenance from metamorphic, igneous, or recycled sedimentary rocks from the Internal Dinarides. Therefore, drastic geomorphic changes must have occurred during the development of the foreland basin, with deep-marine sediments being deposited, subsequently uplifted, eroded, transported, and deposited in continental environments, which will eventually become a new source for a future sink. CITED REFERENCES:Babić, Lj., Zupanič, J. 2008. Evolution of a river-fed foreland basin fill: the North Dalmatian flysch revisited (Eocene, Outer Dinarides). Natura Croatica, 17/4, 357–374.Mrinjek, E., Nemec, W., Pencinger, V., Mikša, G., Vlahović, I., Ćosović, V., Velić, I., Bergant, S., Matičec, D. 2012. The Eocene-Oligocene Promina Beds of the Dinaric Foreland Basin in northern Dalmatia. Journal of Alpine Geology, 55, 409–451.Sinclair, H.D. 1997. Tectonostratigraphic model for underfilled peripheral foreland basins: An Alpine perspective. GSA Bulletin, 109/3, 324–346.

Mass‐transport deposits are products of resedimentation phenomena involving a broad spectrum of gravity‐driven processes, and commonly have a high preservation potential in deep‐marine environments. This study documents various types of mass‐transport deposits that are interbedded with intensely bioturbated shallow‐marine calciclastic sediments deposited along a reflective coast during the middle and late Eocene. The sedimentary succession, located in the vicinity of Novigrad in northern Dalmatia, Croatia, comprises sediments deposited in a range of nearshore and carbonate ramp environments, and represents the infill of a thrust‐top (piggyback) basin of the North Dalmatian foreland basin. Five types of mass‐transport deposits, ranging in thickness from 13 cm to 6 m, have been identified: (i) calcilutite and calcarenite slumps; (ii) conglomeratic slump‐debrites with a ‘dough‐like’ appearance; (iii) blocky‐flow deposits bearing large blocks of beachface and/or shoreface deposits; (iv) rockfall deposits comprising scattered blocks of beachface conglomerates and shoreface calcarenites; and (v) ‘classical’ matrix‐supported debrites. Calciturbidites are rare and mainly comprise Ta and Tb divisions. Conglomeratic slump‐debrites are mostly found in association with offshore‐transition deposits, suggesting that mass flows were triggered above the storm‐wave base likely due to a combined effect of: (i) strong earthquakes related to the tectonic development of the basin; (ii) sediment destabilization due to pore‐water overpressure during forced regressions; and (iii) storm‐wave loading affecting the shallow seabed. Progressive deepening likely favoured mass‐flow transformations, although the overall paucity of turbidites suggests relatively short mass‐flow transport distance and turbidity current bypass to deeper realms. Multiple erosion phases and resedimentation processes from the Cretaceous to the late Eocene contributed to the diverse suite of extraformational clasts in the mass‐transport deposits studied. The mass‐transport deposits may be triggered and emplaced in shallow‐marine settings mainly during regressive stages of basin development, as the diverse gravel clast composition suggests significant tectonic influence. Although the mass‐transport deposits reported herein are relatively small, some of their peculiar sedimentary features and occurrence within shallow‐marine calciclastic deposits render them rather unique and suitable for a re‐assessment of the nature and evolutionary continuum of processes involved in subaqueous sediment mass transport, as well as the preservation potential of sedimentary features in high‐energy wave‐reworked environments.

Mass-transport deposits (MTDs) represent resedimentation phenomena triggered by the combined effect of seismic shocks of regional scale, structural tilting, basin-floor gradient, relative sea-level fluctuations, and/or excess pore-water pressure and can be useful in the reconstruction of basin development dynamics. The present study from the Dinaric Foreland Basin in Croatia documents several limestone blocks (olistoliths), carbonate debris, and associated bipartite carbonate megabeds as MTDs of exotic origin encased in deep neritic hyperpycnites, referred to as host deposits. Detailed facies and micropaleontological analyses indicate that host deposits were sourced from a fluvio-deltaic system located in the proximity of the uplifting orogen, while the MTDs originated from gravitational collapses of late Ypresian and early Lutetian limestones that were uplifted on blind-thrust anticline ridges on the opposite side of the basin. Mass wasting-produced carbonate blocks, debris, and gravity flows were probably triggered concurrently during the middle to late Eocene, but the blocks could have travelled faster downslope due to the lubricating effect of the underlying water “cushion,” overpressured mud, and the pull of gravity. Debrisflows and co-genetic turbidity currents that contributed to the formation of bipartite megabeds were likely mobilized deeper and moved slower than the carbonate blocks and could have been partly deflected by the previously emplaced olistoliths, resulting in megabed thinning along the olistoliths' down-dip edges. Those collapses were most likely triggered by the combined effect of relative sea-level changes associated with tectonic activity and seismic shocks of regional scale. The study suggests that progressive uplift of the frontal blind-thrust anticline ridge resulted in episodic emergence and collapses of progressively older limestone units, and marked the onset of development of the wedge-top basin. Conceptual models of olistolith emplacement and onset of basin development are suggested and may be applicable to both ancient and recent settings. The insights obtained from the integration of detailed facies analysis and micropaleontology may be useful in similar areas where such a level of detail cannot be obtained by conventional field methods.

Gilbert‐type deltas are sensitive recorders of short‐term base‐level changes, but the delta‐front record of a base‐level rise tends to be erased by fluvial erosion during a subsequent base‐level fall, which renders the bulk record of base‐level changes difficult to decipher from the delta‐front deposits. The present detailed study of three large Pleistocene Gilbert‐type deltas uplifted on the southern coast of the Gulf of Corinth, Greece, indicates a genetic link between the delta‐front morphodynamic responses to base‐level changes and the delta‐slope sedimentation processes. Sigmoidal delta‐brink architecture signifies a base‐level rise and is accompanied by a debrite‐dominated assemblage of delta foreset deposits, thought to form when the aggrading delta front stores sediment and undergoes discrete gravitational collapses. Oblique delta‐brink architecture tends to be accompanied by a turbidite‐dominated assemblage of foreset deposits, which are thought to form when the delta‐front accommodation decreases and the sediment carried by hyperpycnal effluent bypasses the front. This primary signal of the system response to base‐level changes combines further with the secondary ‘noise’ of delta autogenic variation and possible allogenic fluctuations in fluvial discharge due to regional climatic conditions. Nevertheless, the evidence suggests that the facies trends of delta foreset deposits may be used to decipher the delta ‘hidden’ record of base‐level changes obliterated by fluvial topset erosion. Early‐stage bayhead deltas may be an exception from the hypothetical model, because their narrow front tends to be swept by river floods irrespective of base‐level behaviour and their subaqueous slope deposits are thus mainly turbidites.

: Results of a detailed sedimentological study of the mid-Pleistocene Gilbert-type Ilias delta at the southern coast of the Corinth Rift, Greece, are reported. The study indicates that the development of a turbidite-dominated facies assemblage (TFA) of delta foreset deposits is associated with an oblique delta-brink geometry, which signifies a deficit of delta-front accommodation due to the falling or stable base level. A debrite-dominated assemblage (DFA) of foreset deposits forms when the base level is rising and the delta-brink geometry becomes sigmoidal, signifying an increase in delta-front accommodation. The comparison of coeval foreset and toeset to bottomset deposits in the delta indicates a reverse pattern of reciprocal changes in facies assemblages, with the foreset TFA deposits corresponding to delta-foot DFA deposits, and with the foreset DFA deposits corresponding to delta-foot TFA deposits. This reciprocal alternation of TFA and DFA facies assemblages is attributed to changes in the delta-front accommodation driven by short-term base-level changes, with a degree of “noise” in the facies record due to the system autogenic variability and regional climatic fluctuations. The study sheds a new light on the changing pattern of subaqueous sediment dispersal in Gilbert-type deltaic systems in response to base-level changes. It also indicates a new possibility for the recognition of a hidden record of such changes in other similar ancient deltas on the basis of their detailed facies analysis.

This study from the southern margin of the Gulf of Corinth documents a Late Pleistocene incised valley‐fill succession that differs from the existing facies models, because it comprises gravelly shoal‐water and Gilbert‐type deltaic deposits, shows strong wave influence and lacks evidence of tidal activity. The valley‐fill is at least 140 m thick, formed in 50 to 100 ka between the interglacials Marine Isotope Stage 9a and Marine Isotope Stage 7c. The relative sea‐level rise left its record both inside and outside the incised valley, and the age of the valley‐fill is estimated from a U/Th date of coral‐bearing deposits directly outside the palaeovalley outlet. Tectonic up‐warping due to formation of a valley‐parallel structural relay ramp contributed to the valley segmentation and limited the landward extent of marine invasions. The valley segment upstream of the ramp crest was filled with a gravelly alluvium, whereas the downstream segment accumulated fluvio‐deltaic deposits. The consecutive deltaic systems nucleated in the ramp‐crest zone, forming a bathymetric gradient that promoted the ultimate growth of thick Gilbert‐type delta. The case study contributes to the spectrum of conceptual models for incised valley‐fill architecture. Four key models are discussed with reference to the rates of sediment supply and accommodation development, and it is pointed out that not only similarity, but also all departures of particular field cases from these end‐member models may provide valuable information on the system formative conditions. The Akrata incised valley‐fill represents conditions of high sediment supply and a rapid, but stepwise development of accommodation that resulted from the spatiotemporal evolution of normal faulting at the rift margin and overprinted glacioeustatic signals. This study adds to an understanding of valley‐fill architecture and provides new insights into the Pleistocene tectonics and palaeogeography of the Corinth Rift margin.