A dominant autocyclic signal on Supergreenhouse carbonates

Abstract

The nature of carbonate cycles and their stacking patterns have been interpreted to be related with global climate conditions through geologic history. Carbonate cycles formed during icehouse times (e.g., late Devonian – early Permian and Neogene) are laterally variable due to allocyclic controls in response to high amplitude, high frequency (fifthand fourth-order) sea-level fluctuations superposed on lower amplitude, low frequency (third-order) eustatic sea-level changes. Whereas cycles formed during greenhouse times (e.g., Mesoproterozoic, Ordovician-early Devonian and late Permian-early Tertiary) are commonly more laterally continuous due to the domination of lower frequency (thirdorder) eustatic sea-level fluctuations. During supergreenhouse climate conditions such as in the Furongian, carbonate cycles were less likely controlled by eustatic sea-level fluctuations. High levels of atmospheric CO2 (>4000 ppm) during this epoch would have prevented the development of polar ice sheet, thus limiting and/or minimizing glacio-eustatic sea-level changes. Tracing the cycles and cycle boundaries in continuous outcrop in the Great Basin, western United States reveals significant lateral variability of cycles. Cycle boundaries disappear within tens to hundreds of meters and component facies of individual cycles pinch out or interfinger with other component facies. Within a cycle set (i.e., cycle stacking patterns bounded by key surfaces), cycle numbers and thickness vary locally and regionally, suggesting dominant autogenic formation of cycles and stacking patterns in the warm Furongian carbonate platforms. Understanding the style of cycles and stacking patterns formation through time has implications to carbonate exploration activities in Southeast Asia and elsewhere. * University of Nevada ** ExxonMobil Indonesia INTRODUCTION About 60% of the world’s oil and 40% of the world’s gas reserves are held in carbonate reservoirs (Schlumberger, 2007). Yet, the very nature of carbonate cycles (i.e., meter-scale cycles and cycle stacking patterns) as the building blocks of these reservoirs remain unpredictable and debated (Spence and Tucker, 2007, Bosence et al., 2009). In a review paper on carbonate cyclostratigraphy, Lehrmann and Goldhammer (1999) concluded that carbonate cycles and their stacking patterns are related to the global climate conditions through geologic history (Figure 1). Icehouse carbonates are laterally variable due to allocyclic controls in response to high amplitude, high frequency (fifthand fourth-order) sea-level fluctuations superposed on lower amplitude, low frequency (third-order) eustatic sea-level changes (Lehrmann and Goldhammer, 1999; Bishop et al., 2010). Whereas greenhouse carbonates are commonly more laterally continuous due to the domination of lower frequency (third-order) eustatic sea-level fluctuations (e.g., Koerschner and Read, 1989; Lehrmann and Goldhammer, 1999; Miller et al., 2003; Saltzman et al., 2004). While giving insight to carbonate cycles studies, these models, however, may have been oversimplified. We hypothesize that carbonate successions formed during supergreenhouse times with high atmospheric CO2 (e.g., Mesoproterozoic, CambrianOrdovician and Cretaceous) should have less influence from glacio-eustatic sea-level changes than those that formed during other times. The Furongian (late Cambrian) carbonate platforms were noticeably formed under the highest atmospheric CO2 levels (~4000–7000 ppm; Berner and Kothavala, 2001) of the Phanerozoic. Such high atmospheric CO2 may have prevented the development of significant continental ice sheets, thus limiting or minimizing glacio-eustatic sea-level fluctuations. Under these conditions, carbonate accumulation was most likely controlled by the © IPA, 2012 35th Annual Convention Proceedings, 2011