MAGYAR ÁLLAMI FÖLDTANI INTÉZE

T

BUDAI, T. 1, NEMETH, K. 1,2 , MARTIN, U. 3, PIROS, O.11. Geological Institute of Hungary, 14 Stefánia út, Budapest, Hungary

2. Eötvös University, Department of Regional Geology, 14 Stefánia út, Budapest, Hungary3. Heidelberg, Germany

Eruptive Mechanism

Middle Triassic Stratigraphy in Western Hungary

This research was supported by grants DFG UM 2334 (UM) OTKA F043346 (KN) and Magyary Zoltan Postdoctoral Fellowship (KN)

SUBAQUEOUS VOLCANICLASTIC SUCCESSIONS IN THE MIDDLETRIASSIC OF WESTERN HUNGARY

Abstract

In western Hungary Middle Triassic sedimentation was steady from the Permian/Triassic till

the end of the Early Anisian that was followed by carbonate platform disruption. Alkaline acidic

volcanism started due to Late Anisian tectonism. The Middle Anisian Lofer cyclic platform

carbonates are sharply overlain by reddish, grey or greenish crinoidal volcaniclastic limestone

with ammonites. This sequence is overlain by a few m thick altered calcareous tuff (“pietra

verde”). These beds are montmorillonitised, bentonitic, with green, yellowish, red matrix

hosting vitro-, litoclasts, and micro-holocrystalline crystals. The K-rich trachyte became

rhyolitic upwards with increasing calc-alkalinity. These beds are thicker in the Anisian basins

(18 m) than above the platforms (5-8 m).

The Upper Ladinian sequence consists of silicified thickly bedded, red, grey, limestone with tuff

layers, and with alternations of tuff, marl and thinly bedded limestone (“posidonia beds”). This

sequence (as Buchenstein Formation) deposited in a pelagic basin, where carbonate

deposition was ended by volcanism. The deposition of this sequence (30 m) occurred during

the Longobardian substage in condensed sedimentation. In contrast in the Southern Alps the

much thicker Upper Ladinian is represented by a volcaniclastic sandstone-silty-marl.

In western Hungary the Upper Anisian to Lower Ladinian volcanics are thick while they are of

subordinate in the Upper Ladinian. Similarity does not exist in the thickness of the sequences

between volcaniclastic rocks of the Lower Ladinian of western Hungary (tens of m) and the

Livinallongo Formation (Dolomites, Italy) (180-200 m). The wide distribution of Lower Ladinian

pyroclastics related to the higher explosivity of the magma and/or subaqueous

reworking/redeposition.

The volcanism became basic and effusive during the Late Ladinian in the Southern Alps. In

Hungary this sequence consists of volcaniclastic sandstones („wengen group”, Southern

Alps). With decrease of silica content of the magma, its viscosity and explosivity decreased

resulting limited dispersal of the deposits.

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Middle Triassic Volcaniclastic Successionsin Western Hungary

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Middle Triassicvolcaniclastic rocks fromwestern Hungary aregenerally thin (dm-scale)units of inverse-to-normalgraded tuff and lapilli tuff.The beds are altered bystrong epidotisation andtheir colour reaches darkgreen with alternatingpinkish fine tuff units. Thevolcaniclastic beds areintercalated withcarbonate mudstoneunits and sandwichedbetween turbidite anddebrite beds of carbonaterich clastic units. Thevolcaniclasts areinferredto have beentransported by turbulentvolcaniclastic gravitycurrents. Themicrotexture of thevolcaniclastic rocks aregenerally well-packedwith angular tosubrounded glassyvolcanic fragments.

Themicrotexture of thevolcaniclastic rocks show agradual transition from aprimary pyroclastic to areworked volcaniclasticcharacter. Rocks that havewell-packed and glassyfragment rich texture inferredto have more prymaryaffinity.

Strong epidote alteration ofjuvenile volcanic materialreplace the original texture ofthe rock completelz in thoselocations where thevolcaniclastic rocks reaches atotal of few dm thickness.

A few cm to a dm thick volcaniclastic units embeddedin pelagic limy turbidite beds are characterisitclyreworked in texture indicated by the abundance ofaltered coloured minerals and the strong abration ofthe glassy mafic clasts. Typical traction featuresindicate that the volcaniclastic interbeds are notsuspension deposited fall units but horizontallytransported volcaniclastic density currents transportedto the pelagic regions by various subaqueous currentsand/or the mechanical energy dispersed by theeruption nearby.

A classic volcaniclastic bedcalled Tc interbedded in apelagic carbonate mud richsiliciclastic turbidite unit(XXXXX). Thevolocaniclastic units bearstextural characterisiticsindicating that it has beendeposited in a similar wayfrom physical point of viewto those beds that areunder- and overlain it. Thisbed documents a phasewhen more volcanic detritusgot introduced to thepelagic depositional systemdue to adistal volcaniceruption.

Inverse-to-normal gradedvolcaniclastic beds represent theclassic pietre verde in the SouthernAlps.

Typical phreatomagmaticsuccession withalternation of tuff andlapilli tuff rich in angularcarbonatic accidentallithic fragments, impactsags, antidunes, scourfillings and tractiondepositional featuresindicate that shallowsubaqueous (Surtseyan)to sunaerial(maar/diatreme)volcanism may havebeen responsible for thegeneration of these unitsin the Southern Alps.

In the centre of the Latemar (Southern Alps,Italy) 4 massive volcaniclastic brecciazones, with fluidisation channels, collapsedwall rock blocks, entrapped fine tuff unitsand semicircular geometry have beenidentified recently and interpreted to bediatremes of phreatomagmatic voclanoes.The link between the dital “pietre verde” andsuch diatreme pipes is not established yet,however a casual link is seemenglyobvious.

Surtseyan model

Near vent position / Latemar vents

Water

Water

Water

Water table

Platform margin slope

Unconsolidated mud

Diagenised platform carbonate

Eruption fed pyroclasticdensity currents

Distal positionWestern Hungarian examples

Distal volcaniclasticturbidites

Maar/diatreme model

Base surge

Volcaniclastic turbidite

Middle Triassic volcaniclastic rocks in WesternHungary are interpreted to be altered maficvolcaniclastic rocks deposited from small volumepyroclastic eruption generated turbidity currents insubaqueous environment. Magma/water interactionis inferred to have been an important fragmentationmechanism in the formation of pyroclasts howeverthe exact transportation mechanism of thevolcaniclasts is diffucult to establish due to thelimited drill core data. A casual relationship betweenthe location of diatreme-like settings, near ventphreatomagmatic pyroclastic successions, anddistal reworked mafic volcaniclastic units identifiedfrom the Southern Alps maybe a good analogy inthe interpretation of the voclaniclastic rocks ofsimilar age in the Western Hungarian realm.

Budai, T., Németh, K., Martin, U. & Piros, O (2004) Subaqueous volcaniclastic successions inthe Middle Triassic of Western Hungary. In: Németh, K, Matrin, U, Goth, K, & Lexa, J (Eds)

21 - 26. September 2004.,Lajosmizse/Kecskemét, Hungary,203: p. 46.

Abstract Volume of the Second International Maar Conference

Occasional Papers of the Geological Institute of Hungary

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Subaqueous volcaniclastic successions inthe Middle Triassic of Western Hungary

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