Preliminary Shipboard Results MV220402: Clinoform Sequence

Stratigraphy in a Modern Foreland

Neal Driscoll, John Milliman, Rudy Slingerland,  JP Walsh, Jeffrey Babcock

The MARGINS Source to Sink program in the Papua New Guinea foreland basin is providing a unique opportunity to determine the genesis and character of foreland stratigraphic sequences as a function of ice-house eustatic sea level fluctuations, high sediment fluxes, and foreland-style tectonic subsidence.  

Although our team is collecting data aboard the R/V Melville as we write this, we already have identified an apparent relationship between relative sea-level curve change and the genesis of the Gulf of Papua clinoform sequences.  Preliminary analysis of 21 piston cores, 24 gravity cores, 1300 nmi of deep-towed CHIRP and more than 2500 nmi of hull-mounted CHIRP sub-bottom profiles in the southern Papua New Guinea foreland basin shows that this shelf is characterized by two distinct stratigraphic units: 1) a modern clinoform on the inner shelf thickening to the north and west whose topsets lie at approximately 20 m water depth and whose foresets extend down to 50-70 m, and 2) an older, as yet un-dated thick sequence of horizontal strata that underlies at least the lower portion of the foresets and extends seaward across the middle shelf (Fig. 1). We interpret this unit as an older clinoform.  This unit has been dissected into cross-shelf trending channels separated by elongate topographic highs.  The little modern sediment that mantles this eroded landscape lies mostly in the topographic lows.

Figure 1. Conceptual sea level subsidence model illustrating clinoform development, erosion and redevelopment in the Gulf of Papau

Based on the configuration of the horizontal strata within the middle shelf clinoform, their stiff character when cored, and their present elevation, we deduce that the mid-shelf clinoform prograded two-thirds of the way across the pre-existing shelf in response to relative sea-level rise during stage 3 and possibly stage 4, when the rate of tectonic subsidence outpaced the rate of sea level fall and the newly created accommodation was roughly equal to the sediment input from the Fly, Kikori, Purari and other coastal rivers.  These may be the first stage 3 clinoforms reported from a continental shelf, and the topsets are 30-40 m thick.  Deflection of the transgressive surface from horizontal yields a post-LGM differential subsidence of about 1 mm/yr from the peripheral bulge to near the basin depocenter.  On the basis of the Chappel and Shackleton eustatic sea level curve, eustatic sea level fell from ~-40 to ~-80  m between 45,000 and 25,000 years BP. During this period, tectonic subsidence must have outpaced eustasy to create the observed 30-40 m of accommodation.

Figure 2. Gulf of Papua showing distribution of different morphologic provinces.

Without firm dates, however, it is also possible that the topsets predate stages 3 and 4, thereby requiring lower rates of tectonic subsidence to create the observed accommodation.  Cut and filled channels within the topset sequence suggest short-term fluctuations in relative sea level, consistent with the Chappel and Shackleton sea-level curve.

The advent of Stage 2 resulted in a rapid eustatic fall commencing ~25,000 years BP that lowered sea level to -120 m, causing incision and headward erosion of streams across the topsets and seaward foresets. The consequent erosion created a

Figure 3. Chirp seismic line collected during our cruise MV220402 imaging the buried erosional landscape along the inner continental shelf.

     landscape with ~20-40 m of relief and a dendritic drainage pattern.  As eustatic sea level rose from -120 to +3 m about 7,000 years BP, the former mid-shelf river channels within the river valley were partially filled, presumably by fluvial and estuarine facies.  The transgression was so rapid however, that the valleys themselves remain unfilled with minimal erosion of the interfluves.  During this rise the river mouths backstepped landward, carbonate sediments began to be deposited on the outer shelf, blanketing the former alluvial plain.  By 7,000 years bp the shoreline was northwestward of its present position, although the precise location remains unknown.  Sediment was sequestered close to shore, thereby preserving the drowned landscape that incised into the mid-shelf clinoform.  From 7,000 years BP to present, sediment supply to the shelf exceeded the creation of new accommodation and the modern clinoform prograded seaward to its present position, progressively covering up the drowned landscape on top of the subjacent clinoform.

Our seismic profiles also provide evidence for the sediment transport processes that are building the modern clinoform.  Much of the upper foreset sequence is composed of discrete sand/mud bars or lobes, which show a preponderance of northeast-dipping reflectors.  These lobes appear to coalesce down the clinoform, presumably the result of downslope gravitational movement and along-slope smoothing by NE-flowing regional currents.  Numerical modeling of circulation and sediment transport for the proposed paleogeographic configurations will help to test these ideas as will more detailed analysis of our piston and gravity cores coregistered with CHIRP profiles.

These preliminary observations suggest that tectonic subsidence typical of foreland basins can generate accommodation during ice-house eustatic sea-level falls because ice-house sea-level fluctuations are highly asymmetric, with declines averaging about 1 mm per year whereas rise rates are an order of magnitude greater.  Analysis of the sediment cores and dating of selected samples should provide evidence to support or modify this conceptual model.