uIntroduction

uFigure captions

uThe upper fan

uComparison with river deltas

uReferences cited

uIntroduction

uFigure captions

uThe upper fan

uComparison with river deltas

uReferences cited

uIntroduction

uFigure captions

uThe upper fan

uComparison with river deltas

uReferences cited

uIntroduction

uFigure captions

uThe upper fan

uComparison with river deltas

uReferences cited

uIntroduction

uFigure captions

uThe upper fan

uComparison with river deltas

uReferences cited

Figure Captions

Figure 1. Dip-oriented seismic line, isochron and interval amplitude maps of the Upper Fan.

Figure 2. Strike-oriented seismic profiles through the Upper Fan; Locations shown in Figure 1.

Figure 3. Horizon slice from 20 ms below top of the Upper Fan showing internal architecture.

Figure 4. Map of sediment concentration of flow simulated for the Upper Fan.

Figure 5. Comparison of Upper Fan stratigraphy with that of Panther Tongue deltas.

Figure 6. Chair display showing the architecture of channel terminus lobe, Upper Fan.

The Upper Fan

The Upper Fan is an 8-km wide, 16-km long, 100-m thick (maximum), SW tapering wedge of sediment capped by a Holocene drape deposited after the fan was abandoned (Figure 1). The fan exhibits a range of seismic facies, a ''shingled'' internal reflection character, and contains both lobate and channel-form depositional elements. These styles of deposits have previously been referred to as Distributary Channel-Lobe Complexes or DLCs (Beaubouef and Friedmann; 2000) and are very sand-rich. The Upper Fan in Basin 4 consists of numerous DLCs arranged in an off-lapping arrangement. DLCs are considered analogous to parasequences in a deep-water setting. Sediment was supplied to the fan from a fixed inlet channel to the northeast. The inlet channel is leveed, but the eastern levee has been removed by erosion and/or slumping. Slumping and large-scale water escape features have modified the eastern margin of the fan related to failure of the over-steepened, eastern basin margin. However, these syn- and post-depositional modifications are relatively minor features that do not significantly impact resolution of the stratigraphic evolution of the fan. The fan is otherwise undeformed. Although some bathymetric control of the deposition is evident, the fan was deposited on a relatively shallow, synclinal ramp that developed after substantial in-filling by earlier deposits reduced the initially steep relief of the basin (Ponded Stage of Beaubouef et al., 2003).

The fan achieves its maximum thickness at a distance of 3-km down-stream of the inlet channel and thins rapidly to the SW from this point. Strong down- and across-fan seismic facies variations accompany these thickness changes and indicate significant lithologic changes within the fan (Figures 2 and 3). Interval amplitude and reflection continuity are correlated inversely with fan thickness. In proximal areas where the fan is thick, the seismic facies are low amplitude and discontinuous (Figure 2A). In distal reaches, the fan is much thinner and characterized by high amplitude and continuous seismic facies (Figure 2C). In proximal and medial regions, across-fan trends show progressive increases in amplitude and continuity from a fan axis position to its margins. The amplitude variability observed is the expression of impedance trends within the fan. The three-dimensional impedance structure of the fan is, in turn, dependent upon the vertical and lateral facies variations. Core data shows that the proximal, low amplitude regions of the fan are sand-rich and lacking in significant interbedded mud-rich intervals. Nested, channel-form reflections and crosscutting erosional surfaces are common in this area suggesting a high degree of amalgamation of sand-filled channels. Hence, reflection continuity is poor in these areas as well. The down-fan increase in interval amplitude is interpreted to reflect the increasing abundance of mud-rich lithologies. Distal, high amplitude regions are interpreted to represent areas of alternating sand and mud that result in pronounced impedance contrasts and high reflectivity. High reflection continuity indicates high continuity of bedding contacts. Although the distal deposits appear highly continuous and sheet-like, closer inspection reveals a high level of complexity in these reaches as well (Figure 3). This general down-fan progression in internal architecture is consistent with that observed in outcrops of ancient submarine fans (Beaubouef et al., 2000). Similar observations can be made of the lateral or across-fan facies changes in the Upper Fan. Together, these characteristics can be understood in terms of the time-averaged behavior of sediment gravity flows entering the basin. The proximal and axial portions of the fan were areas of high velocity, highly concentrated flow and high sedimentation rates. Furthermore, the proximal region of the fan was an area of rapid flow expansion (lateral and vertical) and, hence marked turbulence. In this area deposition and/or preservation of mud would be diminished and erosion would be enhanced giving rise to channelization. Conversely, the distal and lateral portions were areas of lower velocity, less concentrated flow, subdued turbulence and lower sedimentation rates. Here, deposition of the fine-grained components of the suspended load would occur and erosion was relatively minor. 3D computational fluid dynamics is being used to evaluate the depositional trends observed within the Upper Fan (Figure 4; see also Beaubouef et al., 2003). In order to model flows delivered to the fan, we have used the following: 1) the seismically defined paleo-topography of the basal fan surface, 2) knowledge of the slope and depth of the inlet channel, 3) knowledge of the grain size populations within the system, and 4) assumed a medium concentration (5%) of the flow. The map in Figure 4 illustrates the concentration of sand within a flow delivered to the basin early in the depositional history of the Upper Fan. The results show that, despite the significant finer-scale stratigraphic complexity, the depositional processes resulting in the gross or fan-scale variability in lithofacies of the Upper Fan can be modeled very simply.

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Comparisons with River Deltas

As discussed above, the prominent stratigraphic features of the Upper Fan are: 1) the ''shingled'' or off-lapping internal reflection patterns, 2) the down-fan progression in architecture from channel-form elements to more sheet-like elements, 3) the down-fan decrease in sand percent and/or grain size inferred from seismic attributes. In these, and other ways, the stratigraphy of the upper fan is similar to that commonly cited for river deltas. Figure 5 illustrates aspects of these similarities through comparison with a stratigraphic model derived for parasequences of the Cretaceous Panther Tongue deltas (Utah). The seismic traverse through the Upper Fan illustrates the characteristics of the channel-lobe transition and the off-lapping arrangement of the constituent units (DLCs). The longitudinal traverse is comparable to profile DDn in the Panther Tongue model. In both cases, erosional surfaces of the proximal channelized regions shallow and broaden down-stream and pass into relatively conformable bedding surfaces within the inclined forsets of the more distal regions. In the transverse (strike) orientation, the channelized stratigraphy of the inner and outer stream mouth bar settings of the Panther Tongue (AAn, BBn) is analogous to that of the proximal and medial fan settings in Figure 2A and 2B. Likewise, the continuous, sheet-like stratigraphy of the delta front (Figure 4, CCn) is analogous to that of the distal fan setting (Figure 2C).

These stratigraphic patterns record, among other things, the out-building or progradation of the depositional systems through time. Associated with episodes of progradation, a stratigraphic evolution within the Upper Fan is observed in which the fan becomes increasingly channelized and avulsion occurs more frequently through time. This evolution compares favorably that that shown for experimental deltas in Van Wagoner et al. (this volume). Early in the depositional history flows entering the basin experienced minimal interaction with the sea floor and where free to expand and decelerate. The deposits resulting from these flows are lobate bodies that occur immediately down-stream of the inlet channel at a break in slope. These early lobes show only minor internal channelization. Through time, and as deposition continued, flows began to impinge on the aggrading surface of the fan. As a consequence, the flows began to erode the surface of the fan initiating the formation of distributary channels. Additionally, flows were progressively partitioned, or split into multiple channels over time. The advancing depositional front forced the southwesterly translation of the break in slope and flows exiting the channels were forced to pass further into the basin through branching pathways. The channels appear to have filled rapidly after their formation. Diversions of flows due to channel plugging, and the influence of antecedent topography were the primary causes of avulsion. Fan progradation results in the successive off-lap of lobate packages and extension of a distributary network of channels into the basin. At the mouths of the distributaries we find channel terminus lobes that exhibit a stratigraphic evolution similar to that observed for the entire fan, but at a much smaller scale (Figure 6). These lobes are of variable size and thickness and consist of small-scale, fan-like bodies (fans within fans). The lobes also contain distributary networks of small, secondary channels. At the mouths of these smaller distributaries are mouth bars on the order of 10s of meters wide around which the channels bifurcate. These small distributary channels are leveed and channel fills consist of mid-channel bars and point bars. Local crevasse splay deposits adjacent to the trunk channels are also observed. As far as we are aware, this is the first time such an assemblage of deposits has been observed at the termination of a submarine fan. The similarities between these deepwater fan lobes and those of many modern river deltas are remarkable. Preliminary fluid dynamics modeling indicates delivery of sediment to the fan was likely the result of jet-like flow similar to that occurring at the mouths of rivers in flood (Hoyal et al., 2003). These results suggest that fundamental depositional processes and shapes of sedimentary bodies are more independent of depositional environment than previously recognized.

References Cited

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