Environmental changes and shallow marine fossil bivalve assemblages of the lower cretaceous Miyako group, NE Japan

We reconstructed the environmental changes recorded in the Lower Cretaceous Miyako Group via facies analysis and delineated the relationship between depositional facies and the occurrence of diverse marine invertebrate macrofossils. The Miyako Group consists of deposits from alluvial bay-head delta, bayhead delta front, central bay, and lower shoreface to inner shelf depositional settings. Fossil bivalve assemblages responded to shifts in these sedimentary environments. We defined three fossil bivalve assemblages from the central bay and lower shoreface to inner shelf deposits. The assemblages in the inner shelf and central bay deposits are clearly different, even though they occur within similar depositional facies. This contrast in assemblages results from environmental differences between closed and open settings; this interpretation is supported by the occurrence of stenohaline crinoids. We defined a fourth bivalve assemblage in a tsunami deposit intercalated within the bay-head delta front deposits. It consists of polygenic allochthonous shells, some that were derived from an estuarine environment or the shallow seafloor and others that were torn from small reefs.