Reports: UR851198-UR8: Brachiopod Shell Beds as Indicators and Predictors of Ancient Hydrocarbon Seeps: Assessing their Geological, Paleoecological, Evolutionary and Biostratigraphical Significance

Michael Sandy, PhD, University of Dayton

Grant activity included field work to collect primary data, laboratory work to investigate material collected in the field, presentation of results at meetings, and publication of results.

The project was initially set up to identify and study Phanerozoic brachiopod shell beds associated with hydrocarbon seeps. These are primarily monospecies accumulations that are dominated by brachiopods although other taxa may be present (e.g., molluscs). Work so far has focused primarily on the following Mesozoic occurrences. 

1)  Jurassic-Cretaceous, Svalbard:

A newly recorded hydrocarbon seep (“cold-seep”) fauna from the Jurassic-Cretaceous boundary of Svalbard is the subject of on ongoing investigation by colleagues from the University of Oslo, Norway. Interestingly several brachiopod species are present but do not numerically dominate the seep deposits. Therefore this is a quite different occurrence for brachiopods at hydrocarbon seeps. The reasons for this and the paleobiogeographic signature of the brachiopod fauna is under investigation.

A number of genus-level determinations had already been made based on an assessment of the external morphology of the terebratulid and rhynchonellid brachiopods. Laboratory work with undergraduate students has focused on determining the internal characters of some of these brachiopods by preparing serial sections. This is aimed at providing genus- and species-level determinations for the brachiopods. Very good results have been obtained despite some rather strong crystalline infills for the brachiopods; this can make tracing internal structures rather problematic – distinguishing the original brachiopod structures from infilling cements. However, low sedimentation rates in this deeper-water fauna means that the preservation of internal lophophore-support structures is likely to be good (brachidium/loop in terebratulids; crura in rhynchonellids) as the chance of post-mortem damage by infilling sediment is reduced.

2)  Jurassic, Oregon and California:

Lower Jurassic sediments from California and Oregon contain brachiopod-rich carbonate lenses of the rhynchonellid Anarhynchia that are considered to have formed at hydrocarbon seeps in clastic-dominated basinal sequences. A progress report on this research will be presented at the Geological Society of America Annual Meeting in November, 2012. Anarhynchia is noteworthy as the only brachiopod taxon to date that is known from hydrothermal vent and hydrocarbon seep deposits.

3)  Triassic, Romania:

A monospecific occurrence of rhynchonellid brachiopods from the Late Triassic of Romania was studied. Serial sections and acetate peels were taken from a sediment-embedded specimen as the brachiopods proved difficult to extract; however good quality peels were obtained that will be of value for taxonomic purposes. The brachiopod genus in question, Halorella, has recently been shown to have occurred in methane-seep deposits as monospecific brachiopod shell lenses in Oregon. The Romanian brachiopods are associated with an erosional karstic Triassic surface.

4)  Cretaceous, Romania:

A “rediscovery” of specimens of the brachiopod Peregrinella in the collections of the University of Bucharest provided the impetus for this study. Recent field work by colleagues in Romania unearthed additional new material from previously recorded field localities. One locality included Peregrinella in turbidites, in sandy clastics. The conclusion is that these specimens were transported down basin slopes from hydrocarbon seep communities where the brachiopods thrived. This explanation is significant as it provides an alternate ecological model to that proposed by Derek Ager in the 1960’s - that Peregrinella was always transported downslope from rocky shoreline environments where it had lived (yet this original habitat never being preserved). Peregrinella has not been found in contemporaneous shallow water brachiopod faunas suggesting that it does in fact have a unique, or “atypical” paleoecological association. In this case, we concluded that the brachiopods were living in association with hydrocarbon (methane) seeps, and subsequently transported basinward by turbidites. The association with hydrocarbon seeps had previously been reported for localities in California and the Crimea, Ukraine. The Romanian record does strongly suggest a unique paleoecological association for Peregrinella. Following on from this study another occurrence of this brachiopod genus in Tibet is under investigation

5)  Cretaceous, Tibet: A sample from a mass-occurrence of the brachiopod Peregrinella from the Cretaceous of Tibet has been studied. As a result it is now considered to be associated with hydrocarbon seepage during life. This provides a fourth example of Peregrinella from hydrocarbon-seep environments and can be considered the prime brachiopod taxon in such a paleoecological relationship from the Mesozoic. Amongst brachiopods considered to have lived in chemosynthesis-based communities only the Devonian-aged rhynchonellid Dziedusyzckia appears to be known from more localities. In this instance, Dziedusyzckia appears to be known from both chemosynthesis-based communities and more typical “normal marine” brachiopod faunas and is considered by Dr. Jörn Peckmann and colleagues to have had more plastic paleoecological tolerances when compared to Peregrinella – at least based on current results.

6)  Other brachiopod shell beds:

Silurian, Ohio and adjoining states, Midwest of the USA:

Other monospecific brachiopod occurrences have piqued interest during this study which includes horizons dominated by the pentamerid brachiopod Pentamerus in “event-horizons” or “biozones” in SW Ohio in the vicinity of Dayton, Ohio. Although these are not suggested to have originated as a result of hydrocarbon seepage, understanding how these Pentamerus communities developed may lead to some broad understanding regarding the paleoecology of monospecies brachiopod beds.

Jurassic, UK:  

The Early Jurassic Marlstone Rock Bed brachiopod fauna is a brachiopod-dominated horizon that can be traced from SW England to northern Scotland over some 800 km. In places the brachiopod fauna is dominated by 2 species that occur in accumulations that have been called “nests” in the literature. However, the species composition is somewhat variable over the geographic spread of this horizon. Field work, collections study, and laboratory work are aimed at trying to understand the paleoenvironmental significance of this fanua. The Marlstone Rock Bed predates a significant environmental perturbation, the early Toarcian anoxic event. 

A progress report focusing on the brachiopod shell-beds from the Jurassic of California and Oregon and the Cretaceous of Tibet (2 and 5 above) is to be presented with Dr. Jörn Peckmann at the Geological Society of America Annual Meeting in November, 2012. Dr. Jörn Peckmann, University of Vienna is acknowledged for his many valuable contibutions including work on Carbon and Oxygen Isotopes.