Origin of Eocene Depositional Sequences in the Sacramento Basin, California: The Interplay of Tectonics and Eustasy*
Raymond Sullivan1 and Morgan Sullivan2
Search and Discovery Article #50054 (2007)
Posted September 29, 2007
*Adapted from extended abstract prepared for presentation at AAPG Annual Convention, Long Beach, California, April 1-4, 2007
1Department of Geosciences, San Francisco State University, San Francisco, CA ([email protected])
2Chevron Energy Technology Co., Houston, TX
The Eocene strata in the Sacramento basin can be subdivided into six depositional sequences, and each of these can be correlated with a third order regressive-transgressive cycle observed on the global coastal onlap curve. Two types of unconformity bounded sequences are recognized. The first type is associated with submarine canyon formation. These canyons were filled with bathyal shales and turbiditic sandstones, and are represented by the Meganos "C', Capay and Sidney Flat/Kellogg shales. The second type of sequence is associated with predominantly estuarine/fluvial sands. Examples include the Hamilton, Domengine and Ione sands. The close correlation between the chronologic occurrence of these sequences and the global sea level chart suggests a strong eustatic control on the timing of the depositional cycles in the basin. Tectonism, however, is interpreted to have controlled the location of the submarine canyons and the estuarine/fluvial incisements since they stack vertically and follow the tilt of the basin toward the depocenter in the southwest margin of the basin. The interplay of tectonism and relative sea level change also was a determining factor as to which type of depositional sequence formed in the basin. If subsidence rates exceeded the rate of relative sea level fall, less of the shelf was exposed, and submarine canyons were formed. Conversely, when the rate of relative sea level fall exceeded subsidence rates, then the shelf was exposed, and widespread fluvial and estuarine systems developed on the basin margin.
Almgren (1978) recognized that the Paleogene succession in the Sacramento basin is comprised of four marine transgressive/regressive cycles of deposition controlled mainly by tectonism. The total thickness of Paleogene is about 6500 feet (1980m) and each cycle typically is about 1000 feet (305m) thick but they vary greatly depending on their location in the depositional system. This model proposed that the onset of each cycle was preceded by a period of uplift and erosion resulting in the cutting of submarine canyons on the continental shelf and slope. Rapid subsidence and a corresponding sea level rise caused the filling of the submarine canyon with bathyal shale. Subsidence rates eventually lessened and sedimentation continued with the deposition of shallow marine transgressive sandstones above the bathyal shales. A second phase of rapid subsidence followed in the upper part of the cycle with the renewed deposition of bathyal shales overlying the lower shallow marine transgressive sandstone. Shoaling in the latest stage of this marine cycle of deposition resulted in the formation of neritic and marginal marine shale and sandstone at the top of the cycle (Figure 1).
Revised Depositional Sequence Model
The present study modifies the Almgren model and proposes that the Paleogene succession in the Sacramento basin can be placed into a sequence stratigraphic framework. At least six unconformity bounded sequences can be recognized in the Eocene and additional ones exist in the underlying Paleocene succession (Figures 2 and 3). The revised interpretation suggests that Almgren’s depositional cycle is composed of two types of sequences (Figure 1). The first type of sequence is associated with submarine canyon formation in the lower part of Almgren’s cycle. The canyons were eroded during the lowstand and filled with bathyal shale and sand during the transgressive/highstand. These sequences in the Eocene include the Meganos Canyon which is filled with Meganos “C” Shale, the lower Princeton Canyon comprised of Capay Shale fill and the Markley Canyon filled with Sidney Flat/Kellogg shales (Figures 4, 5, and 6). These shales serve as excellent source rocks and seals in the hydrocarbon exploration of the Sacramento basin.
The second type of sequence is equivalent to the upper part of the Almgren’s cycle. The sequence starts at the base of the shallow marine transgressive sands of this cycle. The depositional environment of these sandstones, however, has been reinterpreted as fluvial/estuarine in origin and occupying a lowstand incisement. The sandstones are excellent reservoirs in the basin and are overlain by shelfal/ bathyal shales of the transgressive/ highstand systems tract. Examples of these shallow marine sequences in the Eocene succession include the Domengine-Nortonville and Ione formations.
The present study also shows that Eocene submarine canyon systems are more extensive than previously mapped and all extend into the depocenter in the southern part of the basin (Figure 3). Moreover, the submarine canyons formation is restricted to the Paleogene and does not extend up into the Neogene as suggested by Almgren (1978) and others.
Geological Controls on the Depositional Sequences
Within the available biostratigraphic data for the Eocene in the Sacramento basin, each of these unconformity-bounded sequences can be correlated to the third-order regressive-transgressive cycles observed on the global coastal onlap curve (Figure 3). It has been proposed that this curve was the product of worldwide eustatic changes in sea level (Vail et al., 1977). This relationship between the timing of the Paleogene sequences in the Sacramento basin and the global coastal onlap curve suggests a strong eustatic control on the timing of the development of these depositional cycles. Tectonism, on the other hand, is interpreted to have controlled the location of the submarine and the fluvial/estuarine incisements since they stack vertically and follow the tilt of the basin toward the depocenter in the southwest margin of the basin. The interplay of tectonism and relative sea level change combined to determine which type of depositional sequence formed. If subsidence rates exceeded the rate of sea level fall then relative sea level remained high, and less of the shelf was subaerially exposed as part of the coastal plain. The coastal plain, therefore, would have short fluvial systems but more extensive submarine canyons systems on the shelf and slope. Conversely, when the rate of sea level fall exceeded subsidence rates then large tracts of the shelf would be subaerially exposed during lowstand times. Widespread fluvial systems would be incised on a broader coastal plain at the basin margin.
Regional uplift brought an end to widespread Paleogene marine depositional cyclicity in the Sacramento basin. Post-depositional uplift and erosion played an important part in the extent of preservation of these depositional sequences. The Markley Canyon sequence, being the youngest one in the Paleogene, was most extensively eroded below this regional unconformity. The overlying Neogene is mainly nonmarine in origin and marks the return of volcanic-sourced detritus into the system.
This study proposes that third order sequences can be differentiated in the Paleogene sequences in the Sacramento basin. These unconformity bounded sequences are of two kinds that broadly correspond to the lower and upper parts of Almgren’s depositional cycle. Eustasy controlled the timing of the depositional sequences; tectonism was most important in their location, and both these factors combined to determine the kind of sequence that developed in the basin.
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