Martin Stein | Geology Reports

USGS2006

The 'Orsten': more than a Cambrian Konservat-Lagerstätte yielding exceptional preservation

In several areas of southern Sweden, limestone nodules, locally called Orsten occur within bituminous alum shales. These shales and nodules were deposited under dysoxic conditions at the bottom of what was most likely a shallow sea during the late Middle to Upper Cambrian (ca. 500 million years ago). Subsequently, the name ‘Orsten’ has been referred to particular, mainly arthropod, fossils from such nodules, and, in a wider sense, to the specific type of preservation of minute fossil through secondarily phosphatization. This preservation is exceptional in yielding uncompacted and diagenetically undeformed three-dimensional fossils. ‘Orsten’-type preservation resulted from incrustation of a thin external layer and also by impregnation by calcium phosphate and, therefore, mineralization of the surface of the former animals during early diagenesis. Primarily, this type of preservation seems to have affected only cuticle-bearing metazoans such as cycloneuralian nemathelminths and arthropods. ‘Orsten’ preservation in this sense seems to be limited by size, in having yielded no partial or complete animals larger than 2 mm. On the other end of the scale, even larvae 100 μm long are preserved, often more complete than larger specimens, and details such as setules and pores smaller than 1 μm can be observed. Fossils preserved in such a manner are almost exclusively hollow carcasses, but can be filled secondarily; less common are completely phosphatized compact specimens. The high quality of preservation makes the Swedish ‘Orsten’ a typical Konservat-Lagerstätte. Yet, its special type of preservation is more widespread in time and geographical distribution than assumed initially, and the origin of the phosphate is not necessarily restricted just to one source. Subsequent to the first discoveries of limb fragments of Cambrian arthropods in 1975, animals in this special preservational type have been discovered in several continents and across a broad stratigraphic range including even Proterozoic strata. The latter have yielded early cleavage and metazoan embryonic stages, expanding knowledge on the preservational capacities of the ‘Orsten’. Here, we report the recent status of our research on the ‘Orsten’ and give perspectives for future exploration on a worldwide scale, particularly in light of a recently formed international research group named Center of Orsten Research and Exploration (C.O.R.E.).

Palaeoworld↗

USGS1991

U-series ages of solitary corals from the California coast by mass spectrometry

The purpose of this study is to evaluate the feasibility of dating fossil solitary corals from Pleistocene marine strandlines outside tropical latitudes using the recently developed high sensitivity, high-precision U-series technique based on thermal-ionization mass-spectrometry (TIMS). The TIMS technique is much more efficient than conventional a spectrometry and, as a result, multiple samples of an individual coral skeleton, or different specimens from the same bed can be analyzed. Detached and well-rounded fossil specimens of the solitary coral Balanophyllia elegans were collected from relict littoral deposits on emergent marine terraces along the California coast at Cayucos terrace (elevation 8 m, previously dated at 124 and 117 Ky by α counting), Shell Beach terrace (elevation about 25 m, previously undated), Nestor terrace, San Diego (elevation 23 m, previously dated at 131 to 109 Ky ), Bird Rock terrace, San Diego ( elevation 8 m, previously dated at 81 Ky ). Attached living specimens were collected from the intertidal zone on the modern terrace at Moss Beach. Concentrations of 232 Th in both living and fossil specimens are much higher than in reef-building corals (12 to 624 pmol/g vs. 0.1 to 1.6 pmol/g, respectively). However, because 230 Th/ 232 Th in Balanophyllia elegans are very low (2.22 × 10 −3 to 4.33 × 10 −4 ), the high 232 Th concentrations have negligible effect on the 230 Th- 234 U dates. The high 232 Th concentration in the living specimen (33.1 pmol/g) indicates that a significant amount of 232 Th is incorporated in the aragonitic skeleton during growth, or attached to clay-sized silicates trapped in the skeletal material. The calculated initial 234 U activities in the fossil specimens of Balanophyllia elegans are higher than the 234 U activity in modern seawater or in the modern specimen. The higher initial activities could possibly reflect the influx of 234 U-enriched continental water into Pleistocene coastal waters, or it could reflect minor diagenetic alteration, a persistent and fundamental problem in dating all corals. Samples from a compound specimen from the Cayucos terrace were subjected to different preparation procedures. Samples prepared by a standard acid washing procedure yielded 230 Th- 234 U ages of 125, 123, and 122 Ky, whereas samples prepared by an abbreviated procedure without acid washing yield significantly lower ages of 113 and 112 Ky. Two other specimens from the same bed yielded 230 Th- 234 U ages of 118 and 115 Ky. Also, two specimens from a stratigraphically higher bed yielded ages of 120 and 117 Ky, and three specimens from a lower bed yield ages of 115, 113, and 101 Ky. Nine of the twelve ages of the treated samples from the Cayucos terrace range from 125 to 113 Ky. However, the ages do not follow the stratigraphie order. Two possible interpretations are ( 1 ) the age of the terrace deposit is 125 Ky and all younger ages reflect variable diagenetic alteration or (2) the age of the terrace is 125 to 113 Ky and the ages reflect sediment reworking over a period of 12 Ky. Three specimens from a single bed on the Shell Beach terrace yield ages of 126, 122, and 121 Ky, similar to the older ages from Cayucos. The ages of solitary corals from the Cayucos and Shell Beach terraces are similar to ages of reef-building corals from terraces at numerous tropical localities. These are correlated with the last interglacial sea-level highstand, which probably stood 2 to 10 m above present sea level. The youngest ages and present elevations of the Cayucos and Shell Beach terraces yield tectonic uplift rates of 0.01 and 0.15 m/Ky, respectively, assuming the original elevation of each terrace was 7 m. Four specimens from the basal gravel on the Nestor terrace yielded ages of 145, 143, 137, and 133 Ky. The three oldest ages, however, are older than that associated with the last interglacial. The possible explanations for these older ages are ( 1 ) diagenic alteration or ( 2 ) the Nestor terrace deposits reflect in some way a poorly documented early phase of the last interglacial sea level high stand.

Geochimica et Cosmochimica Acta↗