the Yaquina and Trujillo basins from the Lima Basin to the south. This feature can be seen in SeaMARC II images (Hus- song et al., this volume) in the form of ...
5. S E I S M I C S T R A T I G R A P H I C F R A M E W O R K O F T H E LIMA A N D YAQUINA FOREARC BASINS, PERU1 M a r k W. Ballesteros, 2 Gregory F. Moore, 2 Brian Taylor, 3 and Steve Ruppert 3
ABSTRACT The stratigraphic framework of the Lima and Yaquina forearc basins offshore Peru, determined from multichannel seismic (MCS) data, reveals significantly different Neogene histories in the two basins. Miocene deposition in the Lima Basin was controlled mainly by variations in the relative subsidence rates. Depositional processes, particularly contour currents, may have had a major influence during the Pliocene-Pleistocene. In contrast, the Yaquina Basin shows no evidence of assumed contourites. Severe disruption of the reflectors indicates active tectonism throughout the Neogene in the Yaquina Basin, while reflectors representing late Miocene and younger strata in the Lima Basin are largely undeformed, which indicates relative quiescence. Most sequences in the Lima Basin demonstrate the presence of a hinge line that separates the relatively thin, wedgeshaped landward part from a much thicker, lens-shaped seaward part. Generally, this hinge appears to represent a paleoslope break. Such hinge lines are not evident in the Yaquina Basin. Both basins exhibit migration of the depocenters of the various sequences through time. The movement is to the south and landward in the Lima Basin, while migration is northward in the Yaquina Basin. These migrations appear to be the result of variations in the relative subsidence rates within the basins. In the Lima Basin these movements are closely related to structural features in the basement. In addition to a structural trend that is oriented parallel to the margin, we observed a secondary structural trend that is oriented east-west. Development of structural features along this trend led to the development of two distinct depocenters in most of the stratigraphic sequences in the Lima Basin. INTRODUCTION This paper presents a three-dimensional stratigraphic framework for the Lima and Yaquina basins, based on the interpretation of a closely spaced grid of MCS data obtained during the Leg 112 site survey. Previous efforts were based either on isolated MCS lines (von Huene et al., 1985; Thornburg, 1985) or used single-channel analog data (Thornburg and Kulm, 1981). The Lima Basin is located at a latitude between 10° and 13°S on the upper slope of the Peruvian continental margin (Fig. 1). It is separated from the Salaverry Basin, which is located on the shelf, by a positive basement feature designated the outer-shelf high (Thornburg and Kulm, 1981). The data presented here cover only part of the Lima Basin (Fig. 1). The Yaquina Basin is located at a latitude between 8° and 10°S on the midslope part of the margin (Fig. 1). This basin is separated to the east from the upper-slope Trujillo Basin by a positive basement feature previously designated the upper-slope ridge (Thornburg and Kulm, 1981). Another basement high, oriented at approximately right angles to the trench, separates the Yaquina and Trujillo basins from the Lima Basin to the south. This feature can be seen in SeaMARC II images (Hussong et al., this volume) in the form of fractures and fault scars on the seafloor. METHODS Data acquisition and processing are discussed by Moore and Taylor (this volume). Figures 2 and 3 illustrate the data grids. We identified seismic sequences using the techniques described by Mitchum and Vail (1977) and Mitchum et al. (1977). In the Yaquina Basin, problems with structural disruption and data acquisition (see Moore and Taylor, this volume) greatly reduced 1 Suess, E., von Huene, R., et al., 1988. Proc. ODP, Init. Repts. , 112: College Station, TX (Ocean Drilling Program). 2 Department of Geosciences, University of Tulsa, Tulsa, OK 74104. 3 Hawaii Institute of Geophysics, 2525 Correa Rd., Honolulu, HI 96822.
data resolution, making precise definition of reflector relationships difficult. The limited age constraints for various sequences are based on the Leg 112 drilling results and are supplemented with ages determined from dredge samples (Kulm et al., this volume). In particular, we used data from Sites 679, 682, and 688 (Fig. 2) to project ages in the Lima Basin and results from Sites 683 and 684 (Fig. 3) for estimates in the Yaquina Basin. We constructed a series of isochron and selected time-structure maps for each basin. Fault patterns were simplified or eliminated completely for the sake of clarity. LIMA BASIN We divided the strata in the Lima Basin into 11 seismic stratigraphic units. These units are summarized in Table 1. We selected only the most prominent sequence boundaries. In most cases, the indicated sequences can be subdivided further. The more important attributes of each sequence are discussed next. Sequence L I Sequence LI is composed of parallel reflectors having high amplitude and fair-to-good continuity (e.g., line 14, Plate 1A, in back pocket). Upper reflectors are generally conformable, although truncations can be seen locally in the sequence. No attempt was made to define the base of this sequence because of poor data resolution. Based on similarities to a comparable sequence penetrated in the Yaquina Basin and tentative correlations with Sites 682 and 688, we believe that sequence LI represents middle Eocene deposits. For our purposes, we considered these deposits as acoustic basement. The time-structure map at the top of sequence LI (Fig. 4) shows a relatively even slope on the landward parts. A north/ northwest-trending horst, designated the landward ridge (LR), can be seen in line 14 at 0810 UTC (Plate 1A) and in line 13 at 0100 UTC (Plate 1A). Line 22 runs along the axis of this ridge. Another positive basement feature, called the seaward ridge (SR), trends subparallel to the LR, while line 20 runs along its axis.
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M. W. BALLESTEROS, G. F. MOORE, B. TAYLOR, S. RUPPERT
82°W
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Figure 1. Continental margin off central Peru showing the location of the Lima, Trujillo, and Yaquina forearc basins; modified after Thornburg and Kulm (1981). Note the location of Figures 2 and 3, indicated by the boxes. The SR corresponds to the upper-slope ridge of Thornburg and Kulm (1981). We interpreted the SR as a series of tilted fault blocks rather than a single coherent ridge, based on the sediment truncation patterns in the sequence L2 isochron (Fig. 5). Two subbasins about 400 m deep can be seen in the low area between the LR and SR, relative to an east/west-trending interridge saddle (IRS). Sequence L2 Sequence L2 is characterized by subparallel reflectors of variable continuity and amplitude (e.g., line 14, Plate 1A). These reflectors are broken, discontinuous, and sharply angular to the overlying sequence boundary. Correlation with Site 679 indicates that this sequence is composed of low-energy turbidites of middle Miocene age. Reflectors onlap along the flanks of both ridges, which indicates that these were structural highs during deposition. In many places, the SR lacks sediments of sequence L2, while the LR has a thin but persistent sediment cover (Fig. 5). Numerous faults dissect sequence L2 (e.g., line 14, Plate 1A). Simplified fault patterns (Figs. 4 and 5) indicate that at least two structural trends exist, one approximately north-northwest and almost parallel to the trench, and a subordinate one oriented east-southeast and parallel to the IRS. The sequence L2 isochron map (Fig. 5) indicates two major depocenters. One is on the landward side of the LR, trends eastwest, and has its deepest part at the north end of the study area. The other occurs along the seaward flank of the LR and trends northwest-southeast. Sequence L3 The lower boundary of sequence L3 represents an unconformity surface of early-late Miocene age that spans the interval 78
from 8 to 11 m.y. (Fig. 6; R. von Huene, pers. coramun., 1987). This sequence marks a distinct change in seismic character (e.g., line 14, Plate 1A). It is substantially less disrupted than the underlying strata. Reflectors diverge locally and indicate syn-depositional subsidence. Sequence L3 laps out to the east (seaward of Site 679), which suggests correlation with the latest part of the early-late Miocene hiatus noted at Site 679. The landward part of sequence L3 resembles a wedge, while the seaward part of the sequence is an elongated but more irregular lens. The hinge line, or inflection point, that separates the two areas almost coincides with the landward flank of the SR (Fig. 7). Note the east-west trend in sequence L3, which is superimposed on the more prominent trend parallel to the trench (Fig. 7). Reflector terminations indicate onlap fill and are directed away from the depocenters and toward the intervening thin areas. In the more uniform landward part of the sequence, reflectors onlap toward the east, i.e., landward (Plate 1A, line 14; Fig. 7). Upper terminations are generally conformable, but erosional truncation is evident locally. This indicates that the sequence either was deposited near sea level or was subjected to submarine erosion. Additional evidence in sequence L4 supports the former hypothesis. The landward lapping out of the sequence thus may represent a paleoshoreline. Reflector characters vary greatly, but amplitudes are generally moderate to low, while continuity is fair to good in the landward part of the sequence. Seaward, the reflector character becomes more discontinuous and disrupted with decreasing amplitude. This type of reflector character is typical of shelf deposits (Sangree and Widmier, 1977) and is consistent with other evidence. The disrupted nature of the reflectors probably is related to differential subsidence of the basin. The isochron map of sequence L3 (Fig. 7) shows a well-developed, thickened section that trends north-south and is centered over the SR in the northern part of the study area. Another depocenter is located over the SR near line 23 in the southern part of the study area. This observation indicates nonuniform subsidence of the SR and that the northern and southernmost parts have subsided more rapidly than the central area. The eastward bulge in the seaward truncation of the sequence around line 14 on the isochron map (Fig. 7) and the seaward onlap of reflectors in line 14 at 1100 UTC (Plate 1A) corroborate this observation. Note that the maximum thickness of sequence L3 occurs where the upper-sequence boundary is truncated at the seafloor. This is indicated in the lines north and south of line 14 (Fig. 7). Thus, the rapid thinning seen from this point seaward is partly erosional or nondepositional. Such truncation occurs in many of the Lima Basin sequences (e.g., lines 13 and 14, Plates 1A and IB). Sequence L3 also shows significant thickening in the northeastern part of the study area, which suggests formation of another subbasin. A structural high can be seen in the landward part of the sequence. This is supported by the reversal of onlap directions along line 22 (Plate IC, 0730-0810 UTC) near the intersection with line 13 (Fig. 7). The trend of this high is approximately east-west, which indicates a relationship to the east-west trend in the seaward part of the sequence. The isolated depocenter depicted in Figure 8 (over line 12 between lines 20 and 22) is related to thickening on the north side of the IRS and indicates local reactivation of the faults along this feature during deposition of sequence L3. Sequence L4 As with sequence L3, sequence L4 has wedge-shaped and elongated lens-shaped parts. However, the seaward part of the sequence is much less irregular. The inflection point that marks the transition between the two parts (Fig. 8) represents a paleoslope break and is significantly farther seaward than this same point in sequence L3. Results from Site 679 indicate that the
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