Updating uniformitarianism: stratigraphy as just a set of ‘frozen accidents’

It has long been understood that the stratigraphic record is fragmentary. Barrell was the first to clearly understand the importance of accommodation, and the episodic way in which accommodation is created and removed by geological processes. He demonstrated that typically only a fraction of geological time is represented by accumulated sediment. This point was repeated in influential books by D. Ager. A significant feature of the sedimentary record is the correlation between the duration of a sedimentary unit and its sedimentation rate. Sedimentation rates range over more than eleven orders of magnitude.

The durations of stratigraphic gaps, the distribution of layer thicknesses, and sedimentation rates have fractal-like properties, facilitating the integration of our knowledge of the processes of accommodation generation with data on varying sedimentation rates and the scales of hiatuses and processes operating over all time scales.

This paper proposes the definition of a suite of Sedimentation Rate Scales to encompass the range of time scales and processes in the stratigraphic record. Assignment of stratigraphic units to the appropriate scale should help to clarify preservation mechanisms, leading to more complete quantitative understanding of the geological preservation machine, and a more grounded approach than earlier treatments of stratigraphic completeness.

Comparative taphonomy, taphofacies, and bonebeds of the Mio-Pliocene Purisima Formation, central California: strong physical control on marine vertebrate preservation in shallow marine settings

Background

Taphonomic study of marine vertebrate remains has traditionally focused on single skeletons, lagerstätten, or bonebed genesis with few attempts to document environmental gradients in preservation. As such, establishment of a concrete taphonomic model for shallow marine vertebrate assemblages is lacking. The Neogene Purisima Formation of Northern California, a richly fossiliferous unit recording nearshore to offshore depositional settings, offers a unique opportunity to examine preservational trends across these settings.

Methodology/Principal Findings

Lithofacies analysis was conducted to place vertebrate fossils within a hydrodynamic and depositional environmental context. Taphonomic data including abrasion, fragmentation, phosphatization, articulation, polish, and biogenic bone modification were recorded for over 1000 vertebrate fossils of sharks, bony fish, birds, pinnipeds, odontocetes, mysticetes, sirenians, and land mammals. These data were used to compare both preservation of multiple taxa within a single lithofacies and preservation of individual taxa across lithofacies to document environmental gradients in preservation. Differential preservation between taxa indicates strong preservational bias within the Purisima Formation. Varying levels of abrasion, fragmentation, phosphatization, and articulation are strongly correlative with physical processes of sediment transport and sedimentation rate. Preservational characteristics were used to delineate four taphofacies corresponding to inner, middle, and outer shelf settings, and bonebeds. Application of sequence stratigraphic methods shows that bonebeds mark major stratigraphic discontinuities, while packages of rock between discontinuities consistently exhibit onshore-offshore changes in taphofacies.

Conclusions/Significance

Changes in vertebrate preservation and bonebed character between lithofacies closely correspond to onshore-offshore changes in depositional setting, indicating that the dominant control of preservation is exerted by physical processes. The strong physical control on marine vertebrate preservation and preservational bias within the Purisima Formation has implications for paleoecologic and paleobiologic studies of marine vertebrates. Evidence of preservational bias among marine vertebrates suggests that careful consideration of taphonomic overprint must be undertaken before meaningful paleoecologic interpretations of shallow marine vertebrates is attempted.

Calcareous nannofossils from ODP Leg 192, Ontong Java Plateau

Middle Miocene-upper lower Aptian calcareous nannofossils were recovered from Sites 1183–1187 drilled by Ocean Drilling Project Leg 192 on the Ontong Java Plateau. Nannofossil biostratigraphy indicates the presence of six unconformities among the five Leg 192 sites. These are: (1) between the lowermost Albian and upper middle Albian; (2) between the upper Albian and middle Coniacian; (3) within the lower Maastrichtian; (4) between the lower upper Maastrichtian and basal Danian; (5) within the upper Palaeocene; and (6) between the Oligocene and Miocene.

Previous drilling before Leg 192 on the Ontong Java Plateau and in the SW Pacific (Legs 30 and 130) indicated two episodes for major emplacement of basement during the earliest Aptian and Turonian. The Leg 192 drilling was not able to confirm either of these episodes, but instead indicated that the emplacement of basement on the Ontong Java Plateau was relatively continuous from the latest early Aptian to latest Aptian. Results from Sites 1184 and 1185 indicate the possibility of another magmatic episode during the middle Eocene (Zone NP16).