Abstract
Drowning unconformities and their related strata are important records of key tectonic and environmental events throughout Earth’s history. In the eastern Bird’s Head region of West Papua, Indonesia, Middle Miocene strata record a drowning unconformity present over much of western New Guinea, including several offshore basins. This study records platform carbonate strata overlain by mixed shallow- and deep-water units containing benthic and planktonic foraminiferal assemblages in several outcrop locations across the eastern Bird’s Head region. These heterolithic beds are interpreted as drowning successions that are terminated by a drowning unconformity. We define a succession exposed along the Anggrisi River in the eastern Bird’s Head as a stratotype for carbonate platform drowning in the Bird’s Head, analogous to similar faunal turnovers identified in its offshore basins. Detailed facies analyses, biostratigraphic dating, and paleoenvironmental interpretations using larger benthic and planktonic foraminifera collected from the Anggrisi River succession help to constrain the drowning event recorded onshore as beginning in the Burdigalian and ending in the Serravallian. The cause of platform drowning in the Bird’s Head is attributed to a reduction in the rates of carbonate accumulation due to the presence of excess nutrients in the depositional environment. Already foundering carbonate platforms due to environmental deterioration were left vulnerable to submergence and eventually succumbed to drowning. Low rates of carbonate production were outpaced by the rate of relative sea-level rise caused by high-amplitude oscillations in global glacio-eustatic sea-level change and/or regional tectonic subsidence. The duration of the drowning event across the entire Bird’s Head region is interpreted to have lasted a duration of approximately 9.5 My, between 18.0 and 8.58 Ma. This has implications when interpreting timings of sedimentary basin fill across western New Guinea and in other basins where carbonate platform drowning is recorded.
Highlights
McAdoo and Haebig (1999) described continuous Cenozoic deposition within the North Irian Basin with the exception of a regional depositional hiatus in the Late Miocene. This hiatus is marked by a major unconformity, found at the base of the Mamberamo Formation and the top of the Makats Formation (Fig. 2, Column F), and traceable over large areas of the basin (Visser and Hermes 1962; McAdoo and Haebig 1999). We suggest that this may be a drowning unconformity as it separates shallow-water platform carbonate from overlying deep-water marl and pelagic carbonate
Evidence for mesotrophy in the carbonate production factory observed in the Sungai Anggrisi drowning succession may be one reason for initial carbonate accumulation rates to be low at this location
Outcrops studied in the eastern Bird’s Head are several hundred kilometers away from wells studied in western New Guinea, the similar facies, paleobathymetry, and age of faunal turnover in both regions indicate that carbonate platform drowning was widespread across the entire Bird’s Head region
Summary
The concept of drowning unconformities is well established and there are many examples of carbonate platform drowning strata documented in the rock record worldwide (e.g., Schlager 1981, 1989; Hallock and Schlager 1986; Longman et al 1987; Simone and Carannante 1988; Erlich et al 1990, 1993; Föllmi et al 1994; Drzewiecki and Simo 1997; Weissert et al 1998; Blomeier and Reijmer 1999; Wortmann and Weissert 2000; Wissler et al 2003; Ruiz-Ortiz et al 2004; Mutti et al 2005; Föllmi and Gainon 2008; Sattler et al 2009; Najarro et al 2011; Marino and Santantonio 2010; Brandano et al 2016; Sulli and Interbartolo 2016). Carbonate platforms are defined as flat-topped accumulations of carbonate sediments developed at or very near sea level (Hallock and Schlager 1986).
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