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High-energy, microtidal nearshore deposits and their provenance (Lower Miocene, Burdigalian/Eggenburgian, Alpine-Carpathian Foredeep, Lower Austria)

Nehyba Slavomír, Roetzel Reinhard

Geological Quarterly

2022

Vol. 66, No. 4

art. no. 33

During the Early Miocene (Early Burdigalian/Eggenburgian) marine transgression at the southeastern margin of the Bohemian Massif, gradual flooding occurred along a rocky coast on granitic bedrock of the Thaya Batholith under high-energy, wave-dominated, microtidal and mixed fair-weather and storm conditions. Deposits of the Burgschleinitz Formation overlie a basal unconformity above a subaerial weathered basement surface (transgressive erosional surface) and are interpreted as a transgressive systems tract. The deposits can be divided into four facies associations/depositional environments, i.e., upper-shoreface, foreshore, gravelly beach and backshore/lagoon. Two stages of transgression and successive overtopping of the basement, with different coastal physiographies, were documented. During the initial stage of transgression a barrier island system developed with relatively fine-grained deposits, reflecting the flooding of the distant parts of the Thaya Batholith with a relative flat basement morphology. The subsequent continuation of the transgression led to the flooding of the more proximal parts of the Thaya Batholith with a steeper relief and formation of a rocky shoreline with deposition of gravelly sedi- ments along palaeo-sea cliffs or wave-cut platforms. While gravel clasts of the deposits investigated originate directly from the underlying granites of the Thaya Batholith, provenance studies show that metamorphic rocks of the Moravian Superunit in the hinterland were the main source of sands. This distant source material was probably delivered mainly by small creeks and alluvial fans to the nearshore. Significant differences in heavy mineral composition of the same formation in the wider vi cinity indicate primarily local sources and rapid deposition with subordinate longshore transport, which may reflect a complex coastal palaeogeography. The Lower Miocene deposits of the Burgschleinitz Formation investigated are a rare example of ancient rocky shore deposits, which generally have low preservation potential in the geological record.

Coastal sandy spit deposits (Lower Burdigalian/Eggenburgian) in the Alpine-Carpathian Foredeep of Lower Austria

Nehyba Slavomír, Roetzel Reinhard

Geological Quarterly

2021

Vol. 65, No. 4

art. no. 50

In the type-area of the Eggenburgian regional stage (Lower Burdigalian) sands with large-scale clinoforms were studied north-west of Eggenburg (Lower Austria). Stratigraphic and facies architecture, palaeocurrent pattern and inferred palaeogeographic setting show that these sands are deposits of W-E trending and SW to SSE prograding coastal spit systems, attached to crystalline shoals or islets in the shallow marine Eggenburg Bay. The spits were dominantly formed by shoal parallel accretion above fair-weather wave base due to longshore transport. The 4-5 m thick clinoforms with bottomset, foreset and topset structures contain up to 3.6 m thick, steeply inclined foresets, dominated by sediment gravity-flow deposition. Relatively stable depositional conditions, characterized by strong unidirectional currents, high sand supply and sufficient accommodation space are assumed for their formation. However, internal reactivation surfaces indicate variations in current activity, orientation and velocity. The deposits of spit systems are interpreted as part of a transgressive systems tract. The provenance analysis reveals the local Moravian and Moldanubian crystalline rocks as principal source. The spit sands of the Burgschleinitz Formation show the highest mineralogical maturity within the studied Lower Miocene succession. Intense reworking and redeposition of material from older deposits is evident. Due to the ongoing transgression deposits of the following Gauderndorf Formation and Zogelsdorf Formation exhibit a larger catchment area with input of high amounts of fresh weathered material.