Multiple Dolomitization Episodes In Deep-Water Limestones of the Triassic Lagonegro Basin (Southern Italy): From Early Reflux To Tectonically Driven Fluid Flow

Abstract

Massive dolomitization does not commonly occur in deep-water carbonates. However, this regional study on outcrops and subsurface samples of Triassic Lagonegro basinal carbonates from the southern Apennines fold and thrust belt (Italy) shows that the evolution of the Lagonegro basin was punctuated by two events of massive dolomitization. These processes resulted in two distinct dolomite types, displaying different petrographic and geochemical features. A medium-crystalline and fabric-preservative dolomite in the northern area (Molise, Campagna, and San Fele–Mount Pierno) contrasts with a medium- to coarse-crystalline and fabric-destructive dolomite in the southern area (from Pignola to the high Val d'Agri).\ud \ud Geochemical data (increase in δ18O with respect to typical Late Triassic seawater values, high Sr and low Mn and Fe content, Sr isotopes consistent with Late Triassic seawater) indicate that the dolomite in the northern area is the result of an early replacement caused by a marine and possibly oxidizing fluid during shallow burial. Dolomitizing fluids likely were expelled through reflux from the Norian Apenninic carbonate platform towards the proximal Lagonegro basin. This reflux was promoted by the Norian peculiar paleoceanographic and climatic conditions in the Tethyan region, and affected only the proximal basinal facies.\ud \ud Dolomites in the southern area display a decrease in δ18O compared to Late Triassic seawater values and show lower Sr, higher Mn and Fe contents, and higher Sr isotope ratios compared to the early dolomites of the northern area. Fluid-inclusion microthermometry reveals homogenization temperatures between 70 and 120°C and salinities of 1.9 to 6.4 wt % NaCl eq. for saddle dolomites. These geochemical signatures indicate that the southern area was affected by late diagenetic burial dolomitization, interpreted to be the result of large scale tectonically driven fluid flow during the second compressional phase characterizing the southern Apennines fold and thrust belt emplacement.\ud \ud The dolomite distribution across the Lagonegro basin (northern and southern) is possibly controlled by the paleogeographic distribution of the facies (proximal vs. distal) as well as by their proximity to thrust faults. The Lagonegro case study demonstrates that basinal successions may undergo multiple dolomitization episodes in a manner similar to carbonate platform successions, from early shallow to deep-burial diagenesis. One of the relevant implications is that in areas of restricted oceanographic circulation, during periods of aridity, reflux dolomitization may extend beyond the platform margin, into deep-water carbonates, having the potential to alter their reservoir quality

Secular variations in seawater chemistry controlling dolomitization in shallow reflux systems: insights from reactive transport modelling

“…Fluid flow is driven by the density contrast between the denser, evaporated seawater which develops over restricted areas of the platform top and the normal salinity seawater of the open areas of shelf and adjacent basin plus the interstitial pore water in the subtidal deposits underlying the evaporitic tidal flat/lagoonal deposits. There are also cases reporting dolomites formed by reflux of mesosaline brines and not associated with the precipitation of evaporites, indicating that even a small density contrast may be enough to drive reflux (Simms, ; Whitaker & Smart, ; Melim & Scholle, ; Gabellone et al ., ).…”

Fault‐controlled dolomite bodies as palaeotectonic indicators and geofluid reservoirs: New insights from Gargano Promontory outcrops

“…Such a mechanism has been described recently for the Triassic carbonates of southern Italy by Gabellone et al . (). However, the Lower Cretaceous Apulian platform carbonates do not contain a significant amount of dolomites as is expected in such a model and, again, the model would not explain the isolated dolomite bodies present within the Maiolica Formation.…”

Basin scale evolution of zebra textures in fault‐controlled, hydrothermal dolomite bodies: Insights from the Western Canadian Sedimentary Basin

“…Earlier works suggested that zebra textures form through the preferential dissolution of carbonate or evaporitic sedimentary rocks, where the dark, finely crystalline bands form through replacement of the carbonate host rock and the light, coarsely crystalline bands are cement‐filled cavities (Beales & Hardy, 1980; Fontboté & Gorzawski, 1990; Krebs & Macqueen, 1984; Morrow, 2014). More recent studies, however, have suggested that fracturing is the dominant process by which these cavities are generated (Hiemstra & Goldstein, 2015; López‐Horgue et al, 2009; Nielsen et al, 1998; Swennen et al, 2003; Wallace et al, 1994; Wallace & Hood, 2018), with zebra textures reported in a wide range of compressional (Gabellone et al, 2014; Gasparrini et al, 2006; Iannace et al, 2012; Kareem et al, 2019; Sharp et al, 2010; Vandeginste et al, 2014), strike‐slip (Dewit et al, 2014; López‐Horgue et al, 2010; Swennen et al, 2012) and extensional (Boni et al, 2000; Shelton et al, 2019; Stacey et al, 2021) tectonic settings. A smaller number of studies have also recognized the importance of recrystallization on the formation of these rock textures, at P/T conditions approaching low‐grade metamorphism (Centrella et al, 2022; Kelka et al, 2015, 2017).…”