Monday, September 18, 2017

Formation of the Langhe Hills landscape: The Tertiary Piedmont Basin

At the "end" of the Alpine orogeny, the area that is now the Langhe Hills did not exist in its current form; rather, it was a basin that rested beneath a remnant of the Tethys Sea. This basin -- the Tertiary Piedmont Basin (TPB) -- eventually became the repository for a sedimentary succession -- measuring 3000 sq km -- located at the junction between the southern section of the western Alps and the western termination of the northern Apennines (Mutti, et al., The Tertiary Piedmont Basin) and resting on wedges of both orogens. According to Mutti, et al., the basin rests on a segment of the wedge formed after the Alpine collosional event and was affected by the the growth of the Apennic orogenic wedge from the Oligocene on. Figure 1 below shows the TPB in relation to the major geologic structures of Northern Italy.

Figure 1. Structural sketch map of Northern Italy.
Source: Festa &Codegone, Geological Map of the 
External Ligurian Units ..., Journal of Maps 9, 2013.
The TPB is divided into three sectors -- western (Langhe). central, and eastern -- by the Celle-Sandia and Sestria-Voltaggio fault lines. The sedimentary succession, and underlying basement rocks, are present in all three sectors, as shown in Figure 2A below.

Figure 2: A - Deposits in the basin (Source: The Tertiary Piedmont
Basin, Mufti, et al); B - Age ranges in the Stages of the Miocene
 (Source: Wikipedia); C - Composition and depths of Oligocene
and Miocene deposits (Source: Mufti, et al)

The western sector basement rocks are Brianconnais units -- slices of European crust with a low-grade metamorphic imprint.

The central sector basement rock is of the Voltri Group, slices of oceanic suites composed of ophiolites (small pieces of oceanic crust that have been attached to the continent) and their sedimentary cover metamorphosed at high-pressure - low-temperature. This structure is generally credited with halting the eastern thrust of the Alps. The Sestri-Voltaggio line is the eastern boundary of the Voltri Group.

The eastern sector basement is comprised of Ligurian units, slices of oceanic suites composed of ophiolites and their sedimentary cover metamorphosed at low- to very-low-grade conditions.

Sedimentary Succession: Mid-Oligocene to Miocene
The TPB sedimentary succession began in the mid-Oligocene and continued through the Miocene. Figure 2A shows the population of sedimentations by geographic era while Figure 2C shows the types and extent of sedimentation exclusive of the Messinian period. The deposits through the Tortonian are predominantly terrigenous in natute -- that is, originating from land -- and are primarily sandstones and mudstones. At its deepest points the succession records a thickness of 6000 m (Mutti, et al.).

As the soils of note in the Barolo Zone derive mainly from upper layers of the succession, we will confine the discourse in this post to those layers. Mutti, et al., suggest that the depositional settings evolved as follows:
  • Western sector troughs infilled with mixed terrigenous systems delta-fed from the Western Alps (Cortemalia and Lequio Units)
  • A shallow-marine domain, primarily consisting of southerly fed delta systems (Cessole and Serravelle Units) developed in the central and eastern sectors of the basin
  • A progressive uplift of the eastern and central sectors leading to the final infill of the basin with widespread southerly derived deltaic strata (Serravelle Unit)
  • During the Tortonian, these deltaic sediments experience a sudden regional drowning resulting in the deposition of progressively deeper-water and finer-grained strata (Sant'Agata Fossili Marl) and, eventually, chaotic deposits, indicating a tectonically steepened slope environment
  • These finer-grained slope strata encase re-sedimented coarse-grained and channelized bodies (Vargo Units) which are overlain by mudstones that grade upward to euxinic (black carbon-rich) shales
  • The latter are overlain by Messinian evaporites (not shown in Figure 2C and to be discussed in a later post)
The closure of the Straits of Gibraltar led to the Messinian Unconformity signaling the end of the Oligocene-to-Miocene succession. I will cover the Messinian Salinity Crisis in a follow-up post.

©Wine -- Mise en abyme

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