PALEONTOLOGICAL INTERPRETATIONS OF SYNIMPACT SEDIMENTS FROM THE ICDP-USGS EYREVILLE CORES, CHESAPEAKE BAY IMPACT CRATER

Synimpact diamicton (sediment clasts in a muddy-sand matrix) from the Eyreville A (866.7 m- 443 m depth) and B (1,393 m-1,371 m depth) cores were examined for calcareous nannofossil and palynomorph content. In Eyreville A, the synimpact diamicton shows the mixed-age, mixed-preservation microfossil assemblages that are typical of deposits from the upper part of the Chesapeake Bay impact structure. Counts of calcareous nannofossils show that Cretaceous species form a significant (up to 37%) proportion of the assemblages; this amount contrasts with values from cores located near the outer rim (Langley and Watkins School) where Cretaceous specimens in the diamicton are uncommon. Irregular distribution of the Cretaceous nannofossils downcore suggests chaotic deposition, perhaps related to reflection of waves in a closed basin, and is a different depositional pattern than is recorded from cores located closer to the outer rim. Cretaceous dinocysts are not abundant, but are consistently present, making the Eyreville diamicton samples similar to those of nearby cores (Kiptopeke and Exmore) and distinct from cores near the western outer rim. The deepest productive dinocyst samples (866.1 and 706.7 m) most closely resemble the samples from 278.4 m and 257.6 m in the Langley core in terms of diversity, ages represented, and preservation. Sporadic well-preserved dinocysts with delicate meshwork intact required both long-distance transport and within-clast protection. This deep material differs markedly from the highly fragmented, mixed Eocene-aged, lowest productive sample in the Bayside core (653.7 m) located west of the Chesapeake Bay.

Sediment clasts derived from the nonmarine Cretaceous Potomac Formation are barren of nannofossils. In distinct contrast with the Langley and Watkins cores, larger clasts bearing marine microfossil assemblages are absent. Pollen from Potomac Formation clasts has been identified from 1,086 to 452 m depth. Mixed pollen assemblages in the diamicton were present to at least 862 m depth. Samples from 1,040 m and 485 m depth suggest syndepositional mixing of Zone II to Zone III-age source material within this interval. Sparse, poorly preserved pollen is present from a sandy interval below the granite slab at 1,392 m in Eyreville B; this sand was barren of calcareous nannofossils and dinocysts.