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Description
The Schulz Ranch Sandstone Member of the Williams Formation (Upper Cretaceous) exhibits significant, previously unreported, lateral and vertical variations in the Santa Ana Mountains. Between Trabuco and Black Star Canyons (area I), the Schulz Ranch Sandstone Member consists of five main interbedded lithofacies which comprise a series of poorly to well developed, repetitious, coarsening- and then fining-upward packages ranging from 5 to 50 feet thick. Vertical variations in the type and abundance of lithofacies, and the character of resulting repetitious packages allow division of the Schulz Ranch Sandstone Member into three informal stratigraphic units (1,2,3). Unit 1 gradationally overlies the Holz Shale Member of the Ladd Formation and ranges from 75 to 125 feet thick. The unit consists primarily of interbedded mottled, fine-grained sandstone and cobble conglomerate and was deposited in a storm-influenced, lower shoreface setting. Unit 2 ranges from 60 to 105 feet thick and is dominated by large-scale, repetitious, fining-upward packages (facies E) which reflect the episodic progradation/ abandonment of upper shoreface/ fan-delta lobes. Unit 3 is 40 to 80 feet thick and consists primarily of interbedded mudstone, siltsone, and sandstone that was deposited in upper offshore to lower shoreface environments. Observed lateral variations indicate an overall southward-fining trend for units 1, 2, and 3 in area I. Between Black Star and Coal Canyons (area II), the Schulz Ranch Sandstone Member is divided into three informal stratigraphic units (4,5,6). Units 4 and 5 consist of interbedded sandstone, conglomerate, and mudstone deposited primarily in slope and submarine fan/canyon environments and are laterally equivalent to the upper one-third to one-half of the Holz Shale Member in area I. Unit 6 consists primarily of coarse-grained sandstone and coble conglomerate which was deposited in a shelfal environment and is thought to be correlative to strata of the Schulz Ranch Sandstone Member in area I. In San Juan Canyon (area III), three informal units (7,8,9) within the Schulz Ranch Sandstone Member are gradationally underlain by alluvial fan deposits and overlain by shallow shelf deposits. The Schulz Ranch Sandstone Member in area III represents shelf-bay to upper shoreface/spit environments within an overall retrogradational fan-delta setting.
