Oligo-Miocene tectono-sedimentary evolution of the Langhe Sub-basin: from continental to basinal setting (Tertiary Piedmont Basin - Northwestern Italy)

Abstract

 The geologic and paleogeographic evolution of the Langhe Sub-basin (Tertiary Piedmont Basin) during the Oligo-Miocene was intensely controlled by synsedimentary tectonics. In Rupelian time, the Langhe Sub-basin was dominated by an extensional tectonic regime along high-angle, basement-involving faults generating differentially subsiding, fault-bounded blocks. In Rupelian times, a horst and graben paleotopography developed. From SW to NE, the major paleogeographic elements were: the Rocchetta Cairo Horst, the Borgo Graben, the Spigno Monferrato Horst and the Cartosio Graben. The grabens accommodated thick successions of continental conglomerates of the Molare Fm. A following transgression first reached the Cartosio and Borgo grabens, and only later the adjacent horsts where the marine sands of the Molare Fm rest directly on the crystalline basement. A phase of enhanced regional tectonic subsidence leading to the collapse of the entire southern margin of the Tertiary Piedmont Basin is testified by the onset of deposition of slope to base-of-slope hemipelagic mudstones (Rocchetta Fm) since late Rupelian-early Chattian. This phase of generalized subsidence was accompanied by a change into transtension, mostly along the previous faults. The most prominent intra- Oligocene structural element of the Langhe Sub-basin was the Dego-Spigno Monferrato High, a positive structure of the basement subdivided into secondary horsts and grabens, which strongly controlled thickness and facies of the Rocchetta deposits. Two depocentres, the Turpino and Rocchetta Cairo depocentres, where the Rocchetta Fm reaches thicknesses of about 1000 m, developed to the N and respectively S of the Dego-Spigno Monferrato High. The Rocchetta Fm shows common slump scars, the largest of which, about 1 km wide and up to 150 m deep, in the Molino di Mombaldone area, evolved into a submarine canyon/slope valley system. The largest turbidite bodies occurring in the Rocchetta Fm (Mogliavacca, Brovida, Cobarello and Noceto sandstone bodies) are concentrated in the southern Rocchetta Cairo Depocentre. These bodies are vertically stacked and show a trend of upward increasing width/thickness ratio reflecting a change from upper slope to base-of-slope setting. The largest and youngest is the Noceto unit (lower Aquitanian), infilling a pluri-kilometric half-graben bounded by the normal listric Rio Girosa growth fault. The Rocchetta Fm is capped by hemipelagic and mainly siliceous sediments, named “Montechiaro d’Acqui Siliceous Lithozone” (LS1) (middle-upper Aquitanian) representing a regional marker horizon and interpreted as condensed deposits. This unit forms a single package in the north-eastern area and is subdivided to the SW into minor units (LS1a, LS1b, LS1c) separated by turbiditic formations. In the southern sector a system of NE-trending tectonic lines, the Uzzone Valley Fault System, was active from the middle Aquitanian to the early Burdigalian. The activation of these faults marked a change in tectonic regime, paleogeography and regional stress field of the study area. They are interpreted as growth faults repeatedly activated, showing either extensional/transtensional or transpressional regimes. They generated intrabasinal highs and small-scale slope or base-of-slope basins controlling location and orientation of the turbidite sandstone bodies and their paleocurrent pattern. Specifically, in the middle Aquitanian, the Poggiolo Basin developed and was sealed by deposition of the siliceous LS1b unit. In the middle–late Aquitanian, the Scaletta Uzzone Basin developed and was sealed by deposition of the siliceous LS1c unit. During the latest Aquitanian-early Burdigalian the deposition of the hemipelagic marls of the Montechiaro d’Acqui Fm took place in a slope to base-of-slope environment. In the lowermost part of this formation resedimented glauconitic sandstones and rhodalgal calcarenites (C. Mevie, Pian Bruno and C. Poggi calcarenites), derived from coeval foramol-type carbonate platforms, were deposited. High tectonic mobility in this stage is indicated by the generation of the C. Mazzurini Half-graben, which was infilled with coarse-grained bioclastic sandstones and conglomerates. In the meantime, the Rio Girosa fault was re-activated in compression, leading to the partial inversion of the Noceto Half-graben; moreover, in the southern area strike-slip reactivation of the Uzzone Valley Fault System generated the Rio della Chiesa Basin where the Castelletto Uzzone Sottano and the Rio della Chiesa lower and upper units were deposited. A new cycle started in middle Burdigalian time, with emplacement of coarse siliciclastics (Piantivello unit) into a large base-ofslope valley followed by the widespread deposition of the thin-bedded turbidites and associated sandstone bodies of the Serole Fm. These deposits locally infilled medium- to large-scale slump scars, cut in the underlying Montechiaro d’Acqui Fm. The Serole Fm tapers gradually to the NE and is capped, with onlap relationships, by the thick basinal turbidites of the Cortemilia Fm (upper Burdigalian). The tectonic evolution in the study area is characterized by a progressive change from extensional to strike-slipdominated regime during the time span from the Early Oligocene to the Early Miocene. This changing tectonic regime is thought to reflect the transition from the early Rupelian crustal stretching which occurred in the late stage of the extensional, post-orogenic exhumation of the deep, metamorphic units of the Mesoalpine prism, to the activation, since late Rupelian, of a regional megashear zone between the left-stepping sinistral Villavernia-Varzi Line to the NE and the Stura Fault System to the SW. The wrench faulting is thought to reflect the translation of the Adriatic indenter, with progressive change of motion from NNW-wards to WNW-wards. In the studied area this megashear zone first worked in a transtensional regime from the late Rupelian to the early Aquitanian, and then in alternating transpressional/transtensional regime in the middle Aquitanian-early Burdigalian. During the late Rupelian-early Burdigalian time span, the whole Tertiary Piedmont Basin probably behaved as a strikeslip basin. The upper Rupelian to middle Burdigalian large-scale sandy and conglomeratic turbidite bodies of the Rocchetta, Montechiaro d’Acqui and Serole formations were fed from NW or WNW indicating regional SE or ESE-dipping paleoslopes (in present day coordinates). All these sandstone bodies are characterized by “proximal” coarse-grained turbidite deposits. More distal coeval turbidite systems should have existed downcurrent, but are at present eroded in the uplifted domain of Ligurian Alps. The distal reaches of the Langhe Sub-basin in Chattian to early Burdigalian times are therefore unknown. In any case, it may be suggested that, at least for the largest sandstone bodies of the Rocchetta Fm, the turbidite system most probably extended onto, and possibly beyond, the present-day uplifted Ligurian Alps.