Millimeter-scale phosphatic steinkern preservation is a feature of the taxonomically enigmatic Early Cambrian “small shelly faunas”, but this style of preservation is not unique to the Cambrian; it is ubiquitous, if infrequently reported, from the Phanerozoic record. The polycyclic phosphogenic condensation (PPC) model envisions both the genesis and concentration of phosphatic microsteinkerns as natural outcomes of shell bed genesis through episodic sediment starvation. This model predicts that more reworked and condensed shell bed limestones will contain more phosphatic microsteinkerns, but that even the least reworked limestones may contain some phosphatic particles. We test this model through examination of vertical thin sections densely collected through a 10-meter interval from the classic Cincinnatian (upper Katian, middle Maysvillian North American Stage) upper Fairview Formation, Miamitown Shale, and lower Grant Lake formations at four localities near Cincinnati, Ohio. For each of approximately 50 distinguishable limestone depositional units in each locality, a 2 × 2 cm square was selected for study. Each square was assigned a textural classification (mud content of intergranular space) and a breakage rank (pristine to comminuted). Phosphatic particle distribution was quantified both by visual estimation and by particle counting, with counts ranging from none detected to over 1000 per 4 cm2. Our analyses show a strong positive relationship between phosphate content and both textural maturity and fragmentation. This positive relationship is consistent with the PPC model and confirms that textural maturity can reflect the degree of condensation as well as depth-related environmental energy. This finding suggests that shell bed processes of repeated deposition and reworking make a significant contribution to the generation and accumulation of phosphatic particles. If local-scale sedimentary processes and conditions can control this accumulation, temporal changes in phosphatic sediment deposition rates may be linked to earth changes more complexly than through changing ocean chemistry on a global scale.
|Journal||Palaeogeography, Palaeoclimatology, Palaeoecology|
|State||Published - Dec 1 2019|
Bibliographical noteFunding Information:
Dattilo acknowledges the donors of the American Chemical Society Petroleum Research Fund grant 5525-UR8 for partial support of this research. The final manuscript benefitted from the thorough reviews by two anonymous peer reviewers and guest editor C.R. This paper is a contribution to IGCP 653: The Onset of the Great Ordovician Biodiversification Event.
© 2019 Elsevier B.V.
- Sediment starvation
- Shell beds
- Small shelly fossils
ASJC Scopus subject areas
- Ecology, Evolution, Behavior and Systematics
- Earth-Surface Processes