Sedimentology and chemostratigraphy of a Valanginian carbonate succession from the Baja Guajira Basin, northern Colombia

ABSTRACT: The Kesima Member of the Palanz Formation constitutes the first record of Cretaceous marine sedimentation along the Baja Guajira Basin, northern Colombia. Sedimentologic and petrographic analyses suggest a deposition along a coral reef dominated rimmed carbonate platform. 87Sr/86Sr values between 0.707350 and 0.707400 suggest a Valanginian (136 - 132 Ma) depositional age for the Kesima Member. A positive anomaly on the δ13C values of ~2.2‰ suggests that this rimmed carbonate platform registered the Valanginian Weissert oceanic anoxic event. Although the Weissert oceanic anoxic event resulted on a major drowning of the Circum Tethyan carbonate platforms, it seems to have not affected those from the Circum Caribbean, where several shallow marine carbonate platform successions crop out. The Kesima Member displays a change from an organically produced carbonate factory into an inorganically produced, ooids dominated, carbonate factory during the peak of the Weissert event δ13C anomaly. This change in the carbonate factory, which may represent a major perturbation of the marine carbonate budget along tropical settings during the Weissert event, coincides with a major decrease in global sea level. Finally, the age of the Kesima Member is considerably older than that of other Cretaceous carbonate successions cropping out in other northern South America sedimentary basins (i.e. Perija-Merida, Cesar-Rancheria). Differences in the timing of the Cretaceous marine incursion along northern South America, together with the differences in the Triassic-Jurassic stratigraphy of several sedimentary basins in northern South America, suggest that the Baja Guajira and Maracaibo basins remained as an isolated tectonic block separated from northern South America after the breakup of Pangea.

摘要：哥伦比亚北部下瓜希拉盆地的帕兰茨组凯西马段，是该盆地首个白垩纪海相沉积记录。沉积学与岩石学分析表明，其沉积环境为以珊瑚礁为主的镶边碳酸盐台地。87Sr/86Sr比值介于0.707350至0.707400之间，指示凯西马段的沉积时代为凡今期（136~132 Ma）。δ13C值约2.2‰的正异常显示，该镶边碳酸盐台地记录了魏瑟特海洋缺氧事件。尽管环特提斯洋域的碳酸盐台地因魏瑟特海洋缺氧事件发生大规模淹没，但环加勒比海域的浅海碳酸盐台地序列并未受显著影响，该区域广泛出露多套浅海碳酸盐台地沉积。在魏瑟特事件碳同位素正异常峰值期，凯西马段的碳酸盐沉积体系从有机成因转变为以鲕粒为主的无机成因体系。这一碳酸盐沉积体系的转变或代表魏瑟特事件期间热带海域海洋碳酸盐收支的重大扰动，与全球海平面的大幅下降相吻合。此外，凯西马段的沉积时代远早于南美北部其他沉积盆地（如佩里哈-梅里达盆地、塞萨尔-兰切里亚盆地）产出的白垩纪碳酸盐沉积序列。南美北部白垩纪海侵时序的差异，结合该区域多个沉积盆地三叠纪-侏罗纪地层的差异，表明盘古大陆裂解后，下瓜希拉盆地与马拉开波盆地始终是与南美北部分离的孤立构造块体。