Marco Stefani

Role of autochthonous versus detrital micrite in depositional geometries of Middle Triassic carbonate platform systems

Middle Triassic platforms (Formazione di Contrin, Upper Anisian) in the Italian Dolomites, southern Alps, record large changes in carbonate production and depositional geometry. The changes include variation in the abundance of detrital micrite, i.e., allochthonous calcareous mud, relative to the autochthonous micrite, which formed in situ and was syndepositionally lithified. Carbonate factory dynamics controlled the geometry of the clinostratified slope units and were probably associated with changes in oxygenation. Early Anisian low-relief platforms were followed by late Anisian, high-relief buildups, associated with basinal dysoxic-anoxic environments. The geometric evolution of the platform slopes records a progressive increase in the dip angle of clinostratifications, matched by a deepening evolution of the basinal environments. At the same time, the slope sediments record a gradual change from loose detrital micrite, characterizing the lower portion, to autochthonous micrite, dominating the upper part and recording massive syndepositional lithification. The development of the autochthonous micrite was associated with the preservation of significant amounts of organic matter. This sharp increase in automicrite formation was probably induced by a rapid change from oxic to suboxic conditions in the carbonate slope and margin environments, while anoxic conditions developed in the adjacent organic-rich basin (Moena Formation). The low oxygen level promoted the preservation of organic matter and the activity of sulfate-reducing bacteria, which in turn induced in situ deposition of autochthonous micrite through biomediated processes. This pervasive early cementation and lithification induced the development of steep platform slopes. The conceptual model distilled from this Middle Triassic case can support the interpretation of analogous buildups where the skeletal framework is subordinate to the micrite component.

The Po Delta Region: Depositional Evolution, Climate Change and Human Intervention Through the Last 5000 Years

The Holocene depositional evolution and landform development of the Po River delta area are hereafter illustrated. Eustasy, climate and a growing degree of human intervention largely influenced the delta history. During late phases of the post-glacial transgression, two large estuarine bays developed. About 3000 years ago, the growth of a rectilinear coastline, under high energy meteo-marine conditions, closed the bays. Several generations of wave-dominated delta lobes then prograded into the sea. The modern delta was induced 400 years ago by the digging of an artificial canal and records very fast environmental modification. The present-day framework is largely artificial in nature and subject to a growing degree of environmental dangers, such as river and sea water flooding. Significant landscape values are nevertheless surviving in the region.

ABSTRACT: Carbonate Production, Relative Sea Level Fluctuations and Slope Geometry: Case Histories from the Middle-Triassic of the Italian Dolomites

The understanding of carbonate platform reservoirs can be enhanced through the analysis of outcropping counterparts, such as the ones provided by the study platforms (Cernera and Latemar), particularly rich in both marine cements and microbial boundstones. The birth of both coeval platforms was matched with an anoxic event and with the deposition of a potential source horizon, rich in marine organic matter. The evolution of the short lived Cernera Platform was controlled by a very fast subsidence, badly compensated by its aggrading evolution. The lengthening slopes rapidly steeped up, while the cementation intensity increased. The carbonate production was unable to support the fast platform volume increase and the slope deposits therefore became thinner and thinner, forcing the margin to retrograde; the platform eventually drowned and it was covered by condensed pelagic facies and deep water stromatolite-like structures. The Latemar slope experienced a similar lengthening and steeping up evolution; this carbonate system however grew in a comparatively less subsiding area and it was therefore able to keep and eventually catch up the relative sea level increase. While the platform core shallowed from subtidal environments to cyclic emersions, in the adjacent slopes loose bioclastic and micritic sediments gave place to breccia rich in carbonate cements and microbial boundstones. The subsidence then slowed down considerably enabling the platform to laterally prograde, but no margin are presently preserved from this phase. Both platforms were then at least partially sealed by volcanic and terrigenous deposits and incompletely affected by a permeability enhancing dolomitization.