Steven B. Bachman

Geology and tectonic evolution of a juvenile accretionary terrane along a truncated convergent margin: Synthesis of results from Leg 66 of the Deep Sea Drilling Project, southern Mexico

Drilling results from the Pacific margin of southern Mexico indicate that this region is characterized by a Neogene accretionary wedge, progressively emplaced against older, tectonically truncated continental crust. Accretion has occurred by both offscraping of sediments at the base of the trench slope and underplating of sediments at depth beneath the accretionary wedge and the continental crust. Mass-balance and incremental-uplift studies suggest that about one-third of the incoming sediment is subducted beneath the leading edge of the continental crust. Piston and drill cores indicate that the trench is sand dominated and flanked by slopes covered principally by mud. A large submarine canyon bypasses sediment past the shelf and inner trench slope. The volume of sediment bypassed to the trench and adjacent lower and outer slope equals 5 to 6 times that deposited on the shelf, upper slope, and mid-slope. The muddy inner slope is characterized by foraminiferan-free mud below the calcite compensation depth (CCD) and foraminiferan-bearing mud above the CCD. The upper slope accumulates laminated mud within the oxygen minimum zone. The shelf is covered by sand and mud. Quartzofeldspathic sand compositions in the Leg 66 area reflect sources in the crystalline basement complex exposed along the coast. Structural fabrics of Leg 66 cores from offscraped and overlying slope deposits show zones of inconsistent dip, stratal disruption, and scaly mudstone, characteristics of many melange-wedges exposed on land. Deformation transgresses the boundary between the offscraped and slope deposits, demonstrating tectonic incorporation of the slope sediments into the accretionary wedge. The rate of deformation of the slope deposits diminishes rapidly landward from the trench. Deposits overlying the continental crust show dip patterns due to mesoscopic folding, as well as local spaced cleavage and faulting, but they show no stratal disruption or scaly mudstone Oblique-slip faulting predominates between the accreted wedge and the continental crust and may reflect decoupling of these two basement types. Frozen sediment, probably bearing gas hydrate, was recovered above a bottom-simulating reflector at two sites.

Evolution of a Forearc Basin, Luzon Central Valley, Philippines

The Cenozoic history of the 14 km-thick Luzon Central Valley sequence illustrates the development of a forearc basin. Forearc basins are important both as major sediment traps and as sites of hydrocarbon accumulations. The Luzon basin is floored by oceanic crust on the seaward (western) side and older accreted terranes on the arc (eastern) side. Initial sedimentation on this oceanic crust occurred during early Tertiary northward translation and emplacement of the crust as an ophiolite along a strike-slip or oblique-slip zone. The basal sediments consist of pelagic limestones and thin ash layers overlain by sandy turbidites derived from uplift and progressive dissection of the ophiolite. A sequence of arc-derived sediments at least 26,000 ft (8 km) thick was shed into the astern (arc) side of the basin during late Paleogene to Quaternary convergence along the western margin of Luzon. By the middle Miocene, the Central Valley became a continuous, elongate basin fringed by extensive shelf deposits along both the uplifted seaward and arc sides of the basin. Detritus shed from both flanks filled the subsiding basin and resulted in progressively shallower depths. Nonmarine deposition began in central portions of the basin in the Pliocene and migrated with time both north and south along the basin axis. Late Miocene to Holocene movement along the Philippine fault zone caused uplift and folding of adjacent parts of the basin. Exploration models for the Central Valley predict gas-prone hydrocarbon generation in central portions of the basin at times that coincide with and postdate the formation of both structural and stratigraphic traps. Previous drilling in the basin has either been in areas with thermally immature source rocks or has failed to reach prospective intervals where thermal maturation is inferred. The hydrocarbon potential of the Central Valley has not been determined adequately.

Sandstone petrofacies of the Yager complex and the Franciscan Coastal belt, Paleogene of northern California

Paleogene strata of the Franciscan Coastal belt and the Yager complex (northern California) provide a vivid illustration of the problems encountered during studies of detrital provenance along accretionary continental margins. The northern portion of the Yager complex contains lower percentages of total quartz and higher percentages of feldspar than do correlative stratigraphic units to the south; mean Q-F-L modes are Q-30, F-54, L-16 (N-Yager) and Q-49, F-31, L-20 (S-Yager). A similar spatial trend is evident within the Coastal belt, where mean Q-F-L modes are Q-33, F-47, L-20 (N-CB) and Q-48, F-33, L-19 (S-CB). A third Coastal belt petrofacies (L-CB) is dominated by volcanic-lithic grains. Interpretations of generic provenance for each petrofacies are straightforward. The arkosic sands probably were eroded from deeply dissected granitic plutons, and the more-quartzose sands are attributed to recycling from both orogenic backarc terranes and an adjacent magmatic arc. The influx of volcanic detritus proves that magmatism was active within the drainage basin during Paleogene time. Identification of specific source areas is more problematic, however. Perhaps the north-to-south changes in detrital mineralogy were caused by influx from an Idaho batholith source (arkosic) versus a Sierra Nevada source (quartzose). The depositional paleolatitude of Franciscan sands is unknown, and, in the absence of latitudinal control, potential sources as far south as Mexico remain plausible. Regardless of these uncertainties, the variations common to both Coastal and Yager terranes demonstrate that amalgamation must have preceded any translational events, such that differential slip has not occurred between the two terranes.

Structural Controls on Submarine-Fan Geometry and Internal Architecture: Upper La Jolla Fan System, Offshore Southern California

La Jolla fan, offshore of San Diego, California, is a well-studied example of submarine-fan sedimentation, yet the internal architecture of the fan has remained poorly known. High-resolution seismic data, recorded in a 1 by 2 mi (1.6 by 3.3 km) grid, over much of the fan, allow better understanding of upper and middle fan features and processes, and of structural controls on fan sedimentation. Three bathymetrically prominent conduits supply sediment to the upper La Jolla fan system from stream and nearshore littoral drift-cell sources. La Jolla canyon (and contiguous La Jolla fan valley) is the main feeder to the fan. Seismic profiling data confirm the previously reported erosional character of the channel and constructional nature of flanking levees. These data also reveal that the position of the channel is controlled by the geometry of a buried, hard-rock structure. Newport canyon-channel, northerly feeder to the upper La Jolla fan system, is a single well-defined channel with flanking constructional levees where it lies in a structurally controlled trough. In contrast, the channel splays into multiple broad, poorly defined channels where sedimentation is unconstrained. A buried Pliocene-Pleistocene ancestral version of Newport channel displays the same structural control. The position of Loma sea valley, southern feeder to La Jolla fan system, is tightly controlled by the structure of the steep flank of Coronado Bank. Thus, Loma sea valley trends parallel with the shore and is fed by an orthogonal set of tributary channels which drains the San Diego shelf to the east. At intersections with these tributaries, abnormal thicknesses of sediment partially clog Loma sea valley. These channel fills may be analogous to those occurring in terrestrial trunk stream-intermittent tributary systems like the Colorado River in the Grand Canyon. Seismic data demonstrate that the La Jolla fan system comprises a complex interleaved set of sediment wedges derived from multiple sources and woven around the wrench tectonic fabric of uplifts and basins of the southern California borderland. Thus, La Jolla fan system presents an expansion from the simple radial growth pattern of fan sedimentation to a complex fan system built of a number of smaller interwoven radial growth components. Despite these complexities, lithofacies patterns are in part predictable for the La Jolla fan system. Fault-bounded uplifts form long-lived barriers to sediment dispersal and enhance channel development along their flanks. Multistory channel complexes, detectable seismically, commonly occur in these structurally controlled positions adjacent to wrench elated uplifts.

Sedimentation history and biostratigraphy of ophiolite-related Tertiary sediments, Luzon, Philippines

Pelagic and hemipelagic sediments deposited on the Zambales Ophiolite contain a nearly continuous depositional record of the original setting and emplacement history of this large ophiolite from the late Eocene through the Miocene. Pelagic limestone with thin ash layers (the Aksitero Formation) caps the volcanic complex of the ophiolite along its east flank. Calcareous nannofossil biostratigraphy of this limestone gives sedimentation rates of 3–5 m/m.y. from the late Eocene through the early Oligocene. Rates increase to 10 m/m.y. or more in the upper Aksitero Formation, where sandy turbidites appear in the middle and upper Oligocene sections. Lower Miocene mudstone, sandstone, and conglomerate of the Moriones Formation were deposited at much higher rates; this formation includes channels and debris-flow deposits characteristic of deep-sea fans. Oligocene sandstones are predominantly volcaniclastic, whereas sandstones in the lower Miocene section contain serpentine and other components derived from the ultramafic complex of the ophiolite. Sedimentary facies and sandstone composition show that the Zambales was deeply eroded by the early Miocene and probably first emerged above sea level in the middle or late Oligocene, only 10 to 15 m.y. after it formed as new ocean crust. A comparison of the Aksitero Formation with Deep Sea Drilling Project (DSDP) sites in the western Pacific suggests that the Zambales Ophiolite was originally part of a marginal basin and not an island arc.

Neogene Fore-Arc Basin Development in Northern California: Eel River Basin: ABSTRACT

Strata representing the youngest phase of fore-arc basin development in northern California are exposed in an unusual cross-sectional view across the basin axis. The exposure of a large part of the stratigraphic section of the Neogene Eel River (Humboldt) basin can be attributed to uplift along the northern edge of the Mendocino triple junction. The fore-arc deposits overlie the Mesozoic and Cenozoic accretionary prism and slope deposits of the Franciscan Complex. Outcrop geology of the uplifted southern flank of the basin indicates that the facies and sediment distribution patterns agree with paleobathymetric studies; a complete deep to shallow marine transition is recorded in the basin sediments. Facies studies demonstrate the time-transgressive nature of the sedimentary environments. Proximal facies are landward (east) of coeval deeper water deposits exposed along the coast. The basal contact is clearly depositional on this southern flank of the basin. Sandstones and pebbly conglomerates cut into coastal belt Franciscan accretionary prism sediments inland. A previously undescribed debris flow is conformable on similar Franciscan sediments along the coast. This debris flow is faulted against overlying faulted and fractured basin plain siltstones and fine sandstones (Miocene-Pliocene) which contain thin lenses of shell debris, pebbles, and glauconitic sand. A monotonous accumulation of organic-rich diatomaceous mudstones is capped by amalgamated channels continuing sequences of thin glauconitic sands with locally derived siltstone rip-ups, siltstone, and hemipelagic mudstones. The overlying sediments consist of fine-grained turbidites, thick bioturbated siltstones and fine sandstones, and coarser turbidites. A continental shelf sequence concludes this phase of Eel River basin development. To the south of the main basin outcrop, progressive uplift and faulting related to the migration of the triple junction have left erosional remnants of sediments coeval with Eel River basin rocks. These rocks are found up to 50 km (31 mi) south of the upturned basin edge, suggesting that the basin was at one time more extensive to the south. Shallower depositional environments in some of these basin remnants may indicate the proximity of the original southern edge of the basin. Structural complexities to the south include strike-slip faulting and possible upper Miocene and younger trusting of Franciscan melange over Neogene marine sediments. End_Page 512------------------------------ Shallow gas fields have been developed in the basin; ongoing exploration for deeper oil and gas is not yet definitive. Potentially good source rocks in the deeper parts of the basin, underlying organic-rich Franciscan sediments, and the abundance of potential reservoir rocks higher in the section make this structurally complex onshore/offshore basin an attractive exploration target. End_of_Article - Last_Page 513------------

Seismic Stratigraphy and Sea Level Changes in Active Margin Settings: An Example from Luzon, Philippines: ABSTRACT

Controversy arises when attempting to relate unconformities on a tectonically active margin to global changes in sea level. Seismic stratigraphy studies on active margins generally concentrate on defining large seismic packets and do not directly relate unconformities and their correlative conformities to global sea level changes. In the Central Valley of Luzon, we determined sequence boundaries in the basin and developed an age model that strongly suggests that sea level change is the major factor affecting shorter term (less than 5 m.y.) changes in sedimentation on this active margin. The Central Valley is a Cenozoic fore-arc basin bounded by an arc complex and the left-lateral strike-slip Philippine fault on the east and by an east-dipping subduction zone adjacent to the Manila Trench on the west. Multichannel seismic reflection, well, and outcrop data were used to determine the depositional history of the basin. Because much of the 13 km (8 mi) thick basin fill consists of deep-water marine sediments, conventional criteria such as coastal onlap and erosional truncation could not always be used. Instead, evidence for pulses of submarine fan deposition during lowstands of sea level (suggested by Vail and Hardenbol in 1979, and by Shanmugan and Moiola in 1982) was used to identify some of the sequence boundaries. The ages of the major boundaries, predicted from comp risons with Cenozoic Sea level curves, agreed very well with established ages from well and outcrop data. The supposed difficulty in determining sea level changes in active margins is that tectonic effects override and cloud the effects of global sea level changes. We agree that major regional tectonic events such as the initiation of subduction or strike-slip movement that creates or destroys basin morphologies clearly are the dominant factors in the overall stratigraphy of the basin. However, episodic tectonic events during continued basin evolution result in discrete changes in local basin morphology and sediment source areas which may lead to local unconformities or local increase or decrease in sediment influx. These effects are probably small compared to the basin-wide effects of global sea level changes. Such is the case in the Luzon Central Valley where the effects of global sea l vel changes can be seen throughout the basin. End_of_Article - Last_Page 502------------

Evolution of a Fore-Arc Basin, Luzon, Philippines: ABSTRACT

The Tertiary Central Valley of Luzon is a remarkably well-exposed fore-arc basin, with both the trenchward and arc flanks uplifted and dissected. A study of outcrop geology, drilling results, and seismic reflection records determined the changing geometry of the basin through time, facies distributions, sediment-distribution patterns, and hydrocarbon potential for the basin. The uplifted western flank exposes an ophiolite sequence, pelagic deposits, and deep-sea to shallow-marine clastics that document the emplacement and uplift of the seaward side of the basin. The eastern flank shows non-marine and shallow-marine volcaniclastic aprons shed off the arc, overlain by reefs, shallow-water clastics, and a progressively deepening slope/basin turbidite sequence. Basin geometries and detrital mineralogy suggest that the Central Valley formed as an elongate geomorphic feature in the middle Miocene and progressively subsided and filled into Pliocene time. Over 35,000 ft (10,668 m) of sediment accumulated prior to a westward jump of the fore-arc basin to a site nearer to the Manila Trench. The positions of shelf/slope boundaries and prograding deltas can be documented from seismic-stratigraphic analysis. The history of basin formation and relative positions of possible source and reservoir rocks suggest that the Central Valley has undiscovered hydrocarbon potential. End_of_Article - Last_Page 544------------

Mesozoic and Early Cenozoic Arc-Trench System of California--Do the Pieces Still Fit?: ABSTRACT

The Mesozoic and early Cenozoic arc-trench system of California has been interpreted to include the Franciscan Complex as a subduction complex, the Great Valley sequence as a fore-arc basin, and the Sierra Nevada batholith and volcanic rocks in the western states as the magmatic arc. The similar ages of these elements and the stacking sequence in the Franciscan Complex (youngest to the west) are consistent with this interpretation. However, some recent studies in paleomagnetics, radiometric dating, and sandstone petrology, particularly in the Franciscan Complex, suggest that a simple model of subduction and accretion does not explain many relations. With the growing evidence for lateral translations of microplate along the Pacific margin during the Mesozoic and Cenozoic, some authors have attempted to explain Franciscan complexities by these large-scale lateral translations. Limestone and volcanic blocks in the Franciscan melange have been shown to be allochthonous, but there is yet no definitive evidence for large-scale translations of the melange as a whole. Several problems remain unsolved, including the relation between blueschist terranes, the large time delay between deposition of some sediments and their subsequent metamorphism to blueschist facies, the relation of sandstone provenance between the fore-arc basin and subjection complex, the distribution of sedimentary facies across the entire trench slope/fore-arc region, a d the differing styles of deformation in the various Franciscan belts. Recent studies of modern and ancient arc-trench systems help explain some of these problems, but others remain enigmatic. End_of_Article - Last_Page 893------------