Latitudinal Extent of Rudists Based on Compaction-Corrected Paleomagnetic Results and the Baja BC Hypothesis
Kodama, K P (1), Ward, P D (2), Kirschvink, J L (3) (2)
(1) Dept. of Earth and Environmental Sciences, Lehigh University, Bethlehem, PA 18015 United States; (2) Dept. of Geological Sciences, University of Washington, Seattle, WA 98195 United States; (3) Div. of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125 United States
Abstract:
Paleomagnetic data from the Cretaceous Nanaimo Group sedimentary rocks from Vancouver Island suggest that these rocks have travelled northward 3500 km since approximately 75 ma [Baja BC; Ward et al., 1997]. The paleomagnetically-determined paleolatitude for Nanaimo rocks from Hornby and Texada Islands ($25\deg$) would predict the presence of rudists, marine bivalve mollusks that occupied the same habitats as hermatypic (reef forming) corals of the present day. None have been observed to date in British Columbia. Rudists are known from most tropical localities in the Upper Cretaceous, and their distribution appears to have been regulated by water temperature and/or salinity. When present, rudists are found in abundance. Rudists have been identified at ten localities along the North American west coast from the Rosario Fm. in Baja California to Upper Cretaceous strata cropping out near Gualala in northern California, typically in nearshore marine siltstones, mudstones, and conglomerates. These localities occur on both the Peninsular Ranges (PR) and the Santa Lucia-Orocopia allochthons. In some cases inclination shallowing has been corrected for burial compaction for the units in which rudists have been reported (Point Loma Fm., Williams Fm. [Tan and Kodama, 1998]) or may be inferred from the terrane on which they are located (Salinia-Pigeon Pt. Fm. [Kodama and Davi, 1995]). Based on these corrections rudists were apparently limited to latitudes of less than $\sim$ $38\deg$. If these preliminary results are borne out, the lack of rudists in the Nanaimo Group sediments would suggest that these rocks could have travelled no more than $20\deg$ in paleolatitude and could indicate at least $15\deg$ of inclination shallowing which is similar to that observed for the Pt. Loma Fm. Inclination shallowing in the Nanaimo sediments would have to pre-date the initial cementation of carbonate concretions. Comparison of magnetic anisotropy in recently collected samples from concretions and mudstones will help determine whether the concretions formed before significant burial compaction. An alternative explanation is that the Vancouver Island terrane containing the Nanaimo Group was bathed in an offshore water current system not conducive to rudist growth, or was too deep for them to live.
