William R. Morris

Sedimentologic Evolution of a Submarine Canyon in a Forearc Basin, Upper Cretaceous Rosario Formation, San Carlos, Mexico

The walls, floor, and fill of a submarine canyon are well-exposed near San Carlos, Mexico, in forearc strata of the Upper Cretaceous Rosario Formation. The submarine canyon is about 7 km wide and at least 230 m deep and has eroded a minimum of 150 m into underlying fluvial red beds. It is unclear whether subaerial or submarine processes initiated the canyon cutting; however, marine processes, especially debris flows, modified the morphology of the submarine canyon. The submarine canyon fill and overlying slope deposits form two major fining-upward sequences. The first includes a 120 m thick lower conglomerate-sandstone unit (LCSU) at the base of the canyon fill overlain by a 50-110 m thick middle mudstone-sandstone unit (MMSU). The MMSU consists predominantly of mudstone and thin-bedded sandstone, but includes a channel filled with sandstone beds that form a fining- and thinning-upward sequence. This sequence is overlain by the second major sequence, a 0-60 m thick upper conglomerate-sandstone unit (UCSU), which is confined to three channels within the submarine canyon and passes gradationally upward into slope mudstone. Each of the two major fining-upward sequences records a gradual decrease in supply of coarse-grained sediment to the submarine canyon head. The first fining-upward sequence may correspond to a lowstand and subsequent rise in global sea level or, alternatively, may have resulted from local downdropping of the basin. The second fining-upward sequence does not correspond to global sea level fluctuations but is age-correlative with a drop then rise in relative sea level recognized by other workers 300-400 km to the north in the San Diego-Ensenada area. This sea level drop is inferred to have been a regional-scale tectonic event that affected the forearc basin along its length.

A submarine-fan valley-levee complex in the Upper Cretaceous Rosario Formation: Implication for turbidite facies models

Outcrops of the Upper Cretaceous Rosario Formation along Arroyo San Fernando, Baja California Norte, Mexico, expose a continuous cross-sectional view through a submarine-fan valley-levee complex that is similar in scale to many modern submarine-fan valleys. This unusually complete exposure provides an opportunity to compare an ancient turbidite sequence with modern submarine-fan systems and to test some aspects of submarine-fan facies models. The Arroyo San Fernando submarine-fan valley fill is 670 m thick and 5.5 to 7.5 km wide perpendicular to paleocurrent direction. The valley fill is bordered by >500-m thickness of levee deposits; these levees aggraded with the valley maintaining positive relief over the surface of the fan. The valley fill consists of coarse-grained channel deposits that alternate with fine-grained interchannel deposits. The channel deposits within the valley fill consist of both single (0.25-2 km wide and 10-50 m thick) and amalgamated (as much as 5 km wide and 45-210 m thick) channel deposits. The levee deposits that border the valley fill consist of interbedded sandstone and mudstone beds that are commonly slumped. These slumps slid both toward and away from the valley axis, indicating that slumping was due to positive relief and not solely to undercutting by channels within the valley. The valley system records an overall aggradation, with very little down-cutting of the valley floor or undercutting of the levee walls by channels. The Arroyo San Fernando submarine-fan valley has dimensions comparable to those of modern submarine-fan valleys. The channels within it are comparable in size to many inferred ancient submarine-fan valleys described in the literature and are the same size as channels within modern submarine-fan valleys. These data, together with facies relationships, may indicate that many ancient deep-marine deposits interpreted as submarine-fan valley fill only represent channels within a submarine-fan valley.

ABSTRACT: 3-D Synthetic Seismic Modeling of Turbidite Sandstones

ABSTRACT 3-D synthetic seismic models were generated for a fine-grained, sand-rich turbidite system to illustrate lateral variability in acoustic impedance and seismic response as a function of turbidite facies architecture. The models represent a 250 meter exposed section containing 4 stacked turbidite fan complexes from the Karoo Basin, South Africa. The ultimate goal of this work is to aid in seismic interpretation of deepwater turbidite prospects and reservoirs. Model generation consisted of: 1) construction of the 3-D lithostratigraphic framework from detailed outcrop descriptions and gamma ray measurements, 2) calculation and assignment of rock properties representative of consolidated subsurface reservoir sandstones and shales, and 3) generation of the 3-D synthetic seismic trace volume. Acoustic impedance values were calculated using an empirical relationship to estimate P-wave velocity from porosity and clay volume. Synthetic seismic volumes were generated by convolving the reflectivity function derived from the acoustic impedance volume with Ricker wavelets having different peak-frequencies. These models illustrate: 1) the seismic response to changes in thickness and acoustic impedance of the turbidite channel, lobe and overbank facies without hydrocarbon effects, and 2) the ability to resolve the individual fan complexes using different model pulses End_of_Record - Last_Page 2-------