A New Chronosequence Stratigraphy for the Tertiary of Offshore Sabah and Sarawak, Northwest Borneo, Malaysia

Abstract

Unconformities and condensed sections are boundary events in sequence stratigraphy that represent discontinuities in deposition. These events can be detected using chronostratigraphy. Unconformities, for example, represent periods of geologic time not preserved in the rock record—a hiatus. Condensed sections, however, are thin rock intervals which comprise so much geologic time that they resemble hiatuses. Chronosequences are rock intervals bounded by hiatuses, regardless of their origins, and are the fundamental unit of chronosequence stratigraphy. Graphic correlation of biostratigraphic well data is a practical method of identifying hiatuses and chronosequences in the subsurface (Carney and Pierce, 1995). A graphic correlation plot relates rock thickness to geologic time--vertical thickness vs. horizontal geologic time. The line of correlation, drawn through microfossil datums, depicts geologic time in a rock section. Flat line segments, or terraces, signify periods of geologic time not preserved or highly condensed in sedimentary rock (hiatuses). Oblique line segments between terraces constitute chronosequences. Accumulation rates are directly proportional to the slope of line segments. Graphic correlation analysis of biostratigraphic data from the Upper Eocene-Quaternary section of 100 wells in offshore Sabah and Sarawak has revealed the presence of at least 22 regional hiatuses, H05 to H180, that separate 23 chronosequences, S05 to S190. These hiatuses represent unconformities or highly condensed sections characterized by key microfossil datums (Figures 1-2). The hiatuses occur in both Sabah and Sarawak, and are very similar in timing and duration (Figures 3-4). This suggests that major tectonic/eustatic events during the Tertiary had a regional effect on deposition throughout NW offshore Borneo, and that an integrated Tertiary chronosequence stratigraphy for Sabah and Sarawak is possible. Some Upper Tertiary hiatuses coincide with regional seismic horizons (unconformities) on the Sabah shelf (Bol and van Hoorn, 1980; Levell, 1987) that appear related to tectonism (Meng, 1999; Madon et al., 1999; Balaguru et al., 2003). Graphic correlation, however, has defined additional hiatuses within the Tertiary that are beyond seismic definition. This enhanced stratigraphic resolution reveals that seismic horizons may merge and be misidentified unless verified by microfossils. Maps, basin and depositional models, source rock maturity, and the timing of structural deformation and hydrocarbon expulsion based on miscorrelated seismic horizons will be flawed and misleading. Although regional unconformities are common on the Sabah shelf, they decrease in duration offshore and off-structure and correlate to conformable beds or even condensed sections on highs in the outboard area. Some hiatuses on the shelf, however, may represent widespread condensed sections produced by major eustatic transgressions, such as that of the early Pliocene. Chronosequence stratigraphy, integrated with seismic and geologic data, will be necessary to understand the complex Tertiary structure and stratigraphy of offshore NW Borneo.