GEOCHEMISTRY AND MICROFACIES OF APTIAN ORGANIC RICH BEDS OF PAJA FORMATION AT THE PROXIMITY OF THE SANTANDER MASSIF, EASTERN CORDILLERA OF COLOMBIA: ITS RELATIONSHIP TO OCEANIC ANOXIC GLOBAL EVENTS

Detailed microfacies analyses, TIC (weight % CaCO₃), TOC (weight % C-org) and stable C- isotope measurements (δ¹³C_org) on a 11.67 m section at “Cementos Argos” Quarry (Curiti, Colombia) help to clarify the relationship between the upper Rosablanca Fm, the C_org -rich beds of the lowermost Paja Fm, and Lower Aptian anoxic episodes.

The upper 156 cm of the Rosablanca Fm consists of poorly sorted packstone beds (bivalve/ ammonite; TIC 67.2-93.8%, TOC 0.04-0.6%), wackestone and calcimudstone with fish bones and phosphatic lithoclasts (TIC 61.4 -81.3%, TOC 0.4-0.7%). The topmost 17 cm is a phosphatic and pyritic packstone (TIC 64.6%, TOC 0.16%) with abundant Lucinid bivalves, ammonites, wood fragments and fish bones. δ¹³C_org values show an increasing trend between -24.5 and -22.58‰, punctuated by a negative peak of -24.71‰. The ammonite Nicklesia karsteni (d’Orbigny) indicates an Early Barremian age.

The lowermost 8 meters of The Paja Fm is characterized by intermittent C_org -rich beds of soft, non-bioturbated shale/ calcareous shale, and marlstone (TIC 0.09-50.7%, TOC 0.56-8.84%), and rarely bioturbated, indurated recrystallized calcimudstones (TIC 69.5-80%, TOC 1.16-3.9% to TIC 72.82% - 86.3%, TOC 1.3-1.7%). All beds contain rare calcispheres and fish bones. In contrast concretion levels (TIC>88%, TOC 0.17-0.765%) contain pellets, bivalves and benthic foraminifera.

The overlying 2.1m contains similar lithologies with higher frequency of TIC-rich beds, and recurrence of wackestones, fossiliferous marlstones with bivalves, gastropods and phosphatic remains; they yielded the upper Aptian ammonite Pseudoaustraliceras columbiae (Basse). The δ¹³C_org of Paja Fm shows high constant values beween -22.05 and -20.47‰, with five distinctive lower values (≥-24.06‰) in the concretion samples. These δ¹³C_org values in the lower part of the Paja Fm suggest isotopic interval C7 (Menegatti et al., 1998), a stratigraphic level subsequent to the peak of the Lower Aptian anoxia or OAE-1a.

Early Barremian microfacies of the Rosablanca Fm reveal a change from oxygenated to suboxic conditions on the shallow carbonate platform. The phosphatic bed at the contact between the Formations, the index ammonites, and the temporal variation of the δ¹³C_org suggest a hiatus from the Upper Barremian to the Lower Aptian (~6.4 Ma).