Diethard Sanders
Eislast-Erscheinungen und Intraklasten in der Höttinger Brekzie (Riß-Würm Interglazial) bei Innsbruck (Österreich)
Features of glacial-ice loading and intraclasts within the Hötting Breccia (Riß-Würm interglacial) near Innsbruck (Austria)

Geo.Alp 5, 2008, p. 149–164

Zusammenfassung
In der Höttinger Brekzie (Riß-Würm Interglazial) – einer Abfolge aus verfestigten alluvialen Schuttfächern und versteinerten Schutthängen, die sich an einer südexponierten Bergflanke ablagerten – wurden Eislast-Erscheinungen und verschiedene Arten von Intraklasten beobachtet. Die Höttinger Brekzie wird herkunftsgemäß unterteilt in (a) einen topographisch tieferen Anteil, der von der 'Roten Brekzie' gebildet wird, die auf sanft einfallenden, Gerinne-dominierten alluvialen Schuttfächern zur Ablagerung gelangte, welche durch Erosion von Rotsedimenten und Karbonatgesteinen triassischen Alters gespeist wurden, und (b) einen topographisch höheren Anteil, der von der 'Weißen Brekzie' eingenommen wird, die von vergleichs­weise steilen alluvialen Schutt-Kegeln und von Schutthängen abgelagert wurde, und deren Klastenbestand ausschliesslich von triassischen Karbonatgesteinen gebildet wird. In schlecht zementierten, matrixlosen Lagen der versteinerten Schutthänge der Weißen Brekzie finden sich an vielen Stellen Lithoklasten, die, von Punktkontakten ausgehend, in situ zerbrochen oder zermalmt wurden sowie Frakturen, die vertikal über mehrere Lithoklasten durchsetzen. Diese Frakturmuster bildeten sich infolge der Last des Inntalgletschers wahrscheinlich während der Würm-Eiszeit.
Intraklasten wurden in der Roten und der Weißen Brekzie beobachtet, und werden unterteilt in (1) Einzeitige Intraklasten aus Schutthang-Fazies oder alluvialen Schuttfächer-Fazies, und (2) Mehrzeitige Intraklasten, die gerundete Fragmente von Metamorphiten sowie Einzeitige Klasten in gemeinsamer Matrix eines gelblichen Lime Mudstone enthalten. Zumindest die Mehrzahl der Einzeitigen Intraklasten wurde in voll lithifiziertem Zustand transportiert. Ein Reichtum einiger der Einzeitigen Intraklasten an gerundeten Fragmenten von Metamorphiten, als auch die Matrix der Mehrzeitigen Intraklasten legen Erosion von Grundmoräne des vorhergehenden Glazials (wahrscheinlich der Riß-Vereisung) nahe, während die Weiße und die Rote Brekzie zur Ablagerung gelangten. Die Einzeitigen Intraklasten aus Weißer und Roter Brekzie stammen von lithifizierten Hang-Ablagerungen, deren klastischer Eintrag vom selben Einzugsgebiet wie das der heute erhaltenen Höttinger Brekzie stammt. Dies macht das Auftreten von bisher im Feld nicht mit Sicherheit erkannten intra-sequenziellen Diskordanzen innerhalb der Abfolge der Höttinger Brekzie wahrscheinlich.

Abstract
In the Hötting Breccia (Riß-Würm Interglacial) north of Innsbruck, a well-preserved lithified succession deposited from alluvial fans and talus slopes along a south-facing mountain flank, different types of intraclasts as well as lithoclasts cracked in situ as a result of glacial ice-loading were observed. The Hötting Breccia is traditionally subdivided into a topographically lower part dominated by the '‘Red Breccia' (RB) nourished from erosion of Triassic red beds and carbonate rocks, and deposited from stream-dominated alluvial fans, and a topographically higher ‘White Breccia’ (WB) accumulated from stream-dominated talus fans and talus slopes, and that consists exclusively of lithoclasts of Triassic carbonate rocks. Intraclasts were found both in the Red and White Breccia, respectively. (A) Cracked lithoclasts: In the medial to apical part of lithified talus slopes of the White Breccia, lithoclasts cracked or spalled in situ probably formed upon ice loading during the Würm glacial. Cracked clasts seem to be present only in poorly cemented layers of openwork gravels to cobbles, whereas layers with fine-grained matrix and well-cemented layers of openwork breccias are devoid thereof. Clast fracturation radiated out from point contacts in the subvertical direction among clasts, and in many cases led to pervasive in-situ spallation or complete crushing of clasts into sharp-edged, spiny to platy rock fragments. In many cases, fractures extend vertically over several clasts. Frost action or other pertinent mechanisms of clast fracturation (earthquakes, sediment loading) appear inconsistent with the observed patterns of clast fracturation.
(B) Intraclasts: The intraclasts observed in the Hötting Breccia are subdivided into (1) single-cycle intraclasts that consist of only one type of deposit (talus facies or alluvial fan facies), and (2) multi-cycle intraclasts comprising both single-cycle intraclasts and metamorphic rock fragments embedded within a common matrix of yellow, argillaceous lime mudstone. Single-cycle intraclasts include (a) White-Breccia facies types such as talus breccias deposited from grain flows, breccias of matrix-bearing debris flows, and gravelly stream deposits, and (b) Red-Breccia facies types such as breccias of matrix-bearing debris flows and gravelly-stream deposits, and (c) conglobreccias to conglomerates that typically are rich in rounded fragments of metamorphic rocks, and that are supported by a matrix of yellow, argillaceous lime mudstone. Along the outer boundary of a few single-cycle intraclasts, both truncation of their component lithoclasts by brittle fracture as well as dissolution pits along the outer intraclast boundaries indicate that at least these intraclasts were transported in fully lithified state. For other single-cycle intraclasts with less indicative outer boundaries, transport either in a frozen state or in a lithified state can neither be proven nor discarded. Multi-cycle intraclasts include matrix-supported conglobreccias to conglomerates rich in well-rounded clasts of metamorphic rocks, and with WB and/or RB intraclasts. The matrix of the multi-cycle intraclasts is a slightly argillaceous, yellow lime mudstone. The matrix of the multi-cycle intraclasts perhaps was transported in a firm but plastic state. In some types of single-cycle intraclasts as well as in the matrix of multi-cycle intraclasts, the richness in rounded clasts of metamorphites suggests that glacial till of the preceeding glaciation (probably the Riß Glacial) was subject to erosion during deposition of both the White and Red Breccia, respectively. Erosion of glacial till may have represented a significant source of lime-muddy matrices at least during the early stage of accumulation of the Hötting Breccia. The lithoclast inventory of the single-cycle WB and RB intraclasts indicates that they are derived from the same source area than that which nourished the preserved succession of the Hötting Breccia. The WB and RB intraclasts transported in a fully lithified state however indicate that vestiges of lithified slope deposits were subject to erosion during accumulation of the Hötting Breccia. This, in turn, strongly suggests that the succession commonly subsumed as Hötting Breccia may contain intra-sequence unconformities that to date could not be unequivocally located in the field.


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