A Sequence Stratigraphic Interpretation of the Domengine Formation, Black Diamond Mines Regional Preserve

Ray Sullivan, San Francisco State University, and Morgan Sullivan, ExxonMobil—April 6, 2002

By Dan Day, from the May 2002 NCGS newsletter

Download the field trip guidebook here.

On April 6th the NCGS hosted a field trip to Black Diamond Mines Regional Preserve. Unlike its annual Fall Teachers Day event, this function was planned to discuss the paleoenvironmental conditions under which the Eocene coal-bearing Domengine Formation was laid down. This excellent discussion of the sedimentary processes that molded the deposits exposed at Black Diamond Mines was expertly led by the father-son team of Dr. Ray Sullivan of San Francisco State University and his son Morgan, employed by ExxonMobile Upstream Research Company in Houston, Texas. They were joined by East Bay Regional Parks District mining experts John Waters and Rick Yarborough, who guided members through the maze of underground tunnels. This trip to the Preserve was arranged to present a sequence stratigraphic interpretation of the Domengine Formation that provides a new paleoenvironmental interpretation of its origin.

The Black Diamond Mines region has a rich history. Mining began in 1855 after a local rancher discovered lignite coal while digging on his property. The mining operations grew rapidly, and by the 1870’s, the industry supported 12 mines, 5 settlements, 900 miners, and 3 railways for transporting the coal to docks in Pittsburg. Coal mining declined after 1885, when a better grade coal source was found in Washington State. By 1902, commercial coal production at the mines essentially ceased. In the 1920’s businessman Marvin Greathouse began mining Domengine sand in the Preserve to feed an Oakland glass factory. This also ended after World War II, when cheaper Belgian glass sand used as ship ballast replaced the Black Diamond Mines source. The region is now part of the Black Diamond Mines Regional Preserve, where the East Bay Regional Parks District staff researches its community history, maintains the mining operations, and characterizes wildlife habitats.

Ray and Morgan Sullivan gave a brief introduction to the sedimentary features used to denote various paleoenvironmental interpretations of sedimentary deposits. It is important to note that it is only within the last 30 years that any concerted effort was made to define sedimentary environments based on the spatial and temporal distribution of the associated bedforms, lithologic units, and sedimentary facies. A key exploration tool refined by petroleum geologists in their search for hydrocarbon deposits is sequence stratigraphy. It is the study of genetically related facies (time-equivalent strata) bounded by meaningful chronostratigraphic (time significant) surfaces. Facies refers to time-equivalent sediments of different lithologic composition, and a sequence is defined as a succession of relatively conformable, genetically related strata bounded by unconformities or their correlative conformities. Sequences are composed of parasequences and parasequence sets (successions of parasequences). Sequence boundaries form in response to relative falls in sea level, whereas parasequences and parasequence sets are bounded by marine flooding (transgressive) surfaces. Parasequences consist of distinctive stacking patterns, or successions of sedimentary layers which can differ in composition, grain size, and bedform (structural) characteristics or any combination thereof. It is the association of the various compositional, textural, and structural features of a sedimentary unit over time that can be used to describe the environment where it was formed. The sedimentary environment and its regional architecture are extremely important in defining the reservoir, source rock, and seal layer distribution of an oil field.

Educators working in the coal mine.

After Morgan completed his description of various nearshore marine environments and the sedimentary features that distinguish each, it was time to tour the mine to see the features exposed in the tunnel walls. The trip’s focus was on the Eocene Domengine Formation, a widely recognized unit in the California Coast Ranges that correlates in part with the Ione Formation on the eastern margin of the Sacramento Valley. It is part of the early Tertiary sedimentary strata totaling 6500 feet in thickness at Black Diamond Mines on the northeastern flank of Mount Diablo. The core of this structure is Coast Range ophiolite (sea floor basalt/sediment assemblages) and Franciscan Formation. The sediments mantling this core complex contain radiometrically dated tephra (volcanic ash) layers that confine the uplift of Mount Diablo to less than 3 million years. Other structural evidence suggests a much more recent time interval than this. Although previous investigators interpreted the massif as a piercement structure, NCGS member Ron Crane and others propose an eastward underthrusting (wedge-backthrust) uplift associated with accretionary plate motion along a basal detachment surface (decollement) as the emplacement mechanism. Unruh and Sawyer (1995) offer a third option, a restrained compressive (transpressional) origin caused by stepover transfer of dextral (right-lateral) strike-slip motion from the Greenville fault southeast of Mount Diablo to the Concord fault on its northwest flank. This uplifting action, whatever the source, produced the prominent strike-aligned sandstone ridges and intervening eroded shaley valleys dipping steeply to the northeast. This series of ridges is one of the best continuous exposures of Tertiary sediments in the East Bay. The extensive tunnel network provided Ray and his coworkers a unique opportunity to characterize the paleoenvironment and sequence stratigraphy of the Domengine Formation.

Mesozoic and early Tertiary time had been dominated by a forearc basin in the Sacramento-San Joaquin Valley (Graham et al., 1984), bounded on the west by an active trench-subduction zone complex. The shoreline of this basin was along the current Sierran foothills. During the Tertiary, plate tectonic dynamics shifted from a quiescent volcanic period early to renewed volcanism as plate motion shifted to a transform mechanism in late Tertiary times. Large submarine canyons were periodically cut into the shelf sediments during sea-level lowstands. The Meganos Canyon underlies the southern Black Diamond Mine Preserve and denotes the slow change from a warm, moist subtropical Eocene climate to subsequent cooler, drier conditions in later times. The Domengine Formation flanking Mount Diablo can be traced eastward into the subsurface, where it forms a substantial gas reservoir in the southern Sacramento basin. It lies unconformably on top of underlying strata, and when traced westward, it truncates progressively older lower Tertiary formations until it rests on upper Cretaceous rocks. The early (lower) Tertiary sediments in this area total over 6500 feet thick and comprise a series of transgressive-regressive cycles. Units include nearshore sands, deep-water marine mudstones, and slope turbidites. The Domengine has been interpreted by previous investigators as lagoonal and barrier-beach strata deposited on a north-south trending shoreline. Ray and Morgan’s work, however, which integrates outcrop and subsurface data, indicates a northeast-southwest trending series of incised estuarine valleys oriented almost perpendicular to the prior model as the likely depositional environment. This interpretation has a critical effect on potential petroleum reservoir location in the subsurface Sacramento Valley.

The group was led on a tour of the mine by East Bay Regional Park District mining experts John Waters and Rick Yarborough. Their description of the mining operation was accompanied by geological commentary from Ray and Morgan. The mine excursion clearly illustrated sedimentary features exposed in the drift walls that allowed one to construct a paleoenvironmental model of Eocene conditions at Black Diamond Mines. The bedforms noted in the sediments included mud-draped sigmoidal crossbedding, fining-upward sequences, ripple marks, and limited Ophiomorpha trace fossil burrow bioturbation. The stacking sequence and three-dimensional arrangement of these sediments are consistent with an estuarine environment fluctuating between high-energy tidal conditions and relatively stillwater slack periods that allowed fine muddy layers to deposit atop the sandier bed forms. Although much of the Domengine displays features akin to both estuarine and barrier bar–beach environments, these estuarine systems are oriented parallel to incised coastal valleys that are themselves situated perpendicular to the regional coastline trend.

After the mine tour, the group assembled in a nearby picnic area to partake of a delicious salmon and chicken barbecue prepared by the now legendary Tridib and Mita Guha. Tridib is a former Conoco Oil employee transplanted to the Bay Area from Houston. He is currently principal of Advanced Assessment and Remediation Services in Concord, CA. In Houston he organized many successful field trips for the local geological society. His considerable expertise has been displayed in recent years at the annual NCGS Teachers Day functions held each October at Black Diamond Mines to mark national Earth Science Week. He also plays a key role in helping to plan logistics for NCGS field trips.

Noontime barbecue attended to by NCGS officers (from left) Phil Reed (Treasurer), Dan Day (Secretary), and Tridib Guha (Field Trip Coordinator).

Lunch was followed by a vehicle caravan excursion eastward across the Preserve to examine many of the features that had been pointed out earlier in the subsurface. The Domengine at Black Diamond Mines represents the lower of two sequences that constitute the formation. Sequence 2, missing at the mines, occurs in the subsurface to the east. The group drove up the road from Somerville to Nortonville to the east, passing Rose Hill cemetery, and descending the ridge to the next valley at the mouth of Coal Canyon. Following this canyon to the south, the contact (transgressive surface 1) between the upper (marine shales) and lower members of the Domengine can be seen. Channelized subtidal sandstones of the lower member are exposed at the canyon mouth, and the Clark coal vein crops out about 70 feet below this contact. The upper member coarsens upward into tabular sandstone beds. Ray took the group to a nearby outcrop that showed the contact between the Domengine and the overlying Nortonville Shale, which forms the valley between the Domengine ridges and the more resistant sandy layers of the ridge-forming Markley Formation to the south. The Domengine-Norton transition marks a major flooding event and a rapid return to bathyal conditions in the area. Time constraints forced the caravan to make the next stop at the western end of the preserve its last. There Ray and Morgan discussed the exposed unconformity between fluvial conglomerates of the basal Domengine and the underlying lower Eocene Meganos “C” shale.

The NCGS is deply indebted to long-time member Dr. Ray Sullivan of San Francisco State, and his son Morgan, for presenting a look at the detailed stratigraphy of the Domengine Formation at Black Diamond Mines Regional Preserve. Ray has devoted a significant portion of his 40-year study of the Tertiary units exposed in the Preserve to characterizing their paleoenvironments. The advent of sequence stratigraphic concepts helped him determine that the Domengine was laid down under estuarine rather than barrier bar–beach conditions. This revelation has significant implications for subsurface gas exploration efforts in the Sacramento Valley east of the Coast Ranges. Ray has long been assisted in his work by Regional Park District mining expert John Waters. John and assistants like Rick Yarborough have painstakingly restored the mines and converted them into a unique classroom for illustrating mining techniques to the public. The NCGS also thanks Phil Reed, Ron Crane, and Dan Day for helping Tridib and Mita Guha with the barbecue. Ron supplied expert commentary on the regional geology of the area to tie in with the local geology at Black Diamond Mines.