Stratigraphic and climatic implications of clay mineral changes around the Paleocene/Eocene boundary of the northeastern US margin

Abstract

Kaolinite usually is present in relatively small amounts in most upper Paleocene and lower Eocene neritic deposits of the northern US Atlantic Coastal Plain. However, there is a short period (less than 200,000 k.y.) in the latest Paleocene (upper part of calcareous nannoplankton Zone NP 9) when kaolinite-dominated clay mineral suites replaced the usual illite/smectite-dominated suites. During this time of global biotic and lithologic changes, kaolinite increased from less than 5% of the clay mineral suite to peak proportions of 50–60% of the suite and then returned to less than 5% in uppermost Paleocene/lowermost Eocene strata. This kaolinite pulse is present at numerous localities from southern Virginia to New Jersey. These sites represent both inner and middle neritic depositional environments and reflect input from several river drainage systems. Thus, it is inferred that kaolinite-rich source areas were widespread in the northeastern US during the latest Paleocene. Erosion of these source areas contributed the kaolinite that was transported and widely dispersed into shelf environments of the Salisbury embayment. The kaolinite increase, which occurred during a time of relatively high sea level, probably is the result of intensified weathering due to increased temperature and precipitation. The southern extent of the kaolinite pulse is uncertain in that uppermost Paleocene beds have not been identified in the southern Atlantic Coastal Plain. The late Paleocene kaolinite pulse that consists of an increase to peak kaolinite levels followed by a decrease can be used for detailed correlation between more upbasin and more downbasin sections in the Salisbury embayment. Correlations show that more upbasin Paleocene/Eocene boundary sections are erosionally truncated. They have varying portions of the kaolinite increase and, if present at all, discontinuous portions of the subsequent kaolinite decrease. As these truncated sections are disconformably overlain by lower Eocene strata, rapid erosional removal of large parts of the most kaolinite-rich P/E boundary clay deposits occurred by early Eocene time. Erosion of the kaolinite-rich P/E boundary beds was enhanced during times of sea-level fall when kaolinite-rich sediments were redeposited to produce kaolinite spikes in basal beds of lower and middle Eocene sequences that have little or no kaolinite elsewhere in the sequence. In contrast, more downbasin sites document only the upper, decreasing part of the kaolinite pulse. The absence of strata documenting the earlier kaolinite increase is attributed to slow sedimentation (condensed interval) as a result of a significant sea level rise that ponded most sediments in shallower waters, combined with the probable subsequent erosional removal of these thin downbasin deposits by oceanic currents.

Introduction

Many latest Paleocene atmospheric, biotic, climatic, and oceanographic events have been suggested, including: (1) changes in deep-water circulation and the possible conversion to primarily salinity-driven, poleward deep-ocean circulation (Rea et al., 1990, Pak and Miller, 1992, Schmitz et al., 1996); (2) reduction in the intensity of atmospheric circulation (Janecek and Rea, 1983, Rea et al., 1990, Hovan and Rea, 1992); (3) global warming, especially at high latitudes (Kennett and Stott, 1991, Robert and Kennett, 1994), and a decrease in latitudinal temperature gradients (Zachos et al., 1994, Greenwood and Wing, 1995); (4) increased evaporation at low latitudes and increased precipitation at higher latitudes (Rea et al., 1990, Robert and Chamley, 1991, Robert and Kennett, 1992, Robert and Kennett, 1992); (5) rapid evolutionary and/or environmental turnover or dispersal of various biotic groups, including deep-sea and neritic benthic foraminifers (Tjalsma and Lohmann, 1983, Miller et al., 1987, Thomas, 1990, Gibson et al., 1993, Thomas and Shackleton, 1996, Speijer et al., 1997), calcareous nannofossils (Gibson et al., 1993), land mammals (Rea et al., 1990, Hooker, 1996), and land plants (Gibson et al., 1993, Frederiksen, 1994, Wing et al., 1995, Wing, 1998); (6) fluctuations in the terrestrial and marine carbon isotope record (Kennett and Stott, 1991, Koch et al., 1992, Stott, 1992, Schmitz et al., 1996, Stott et al., 1996, Thomas and Shackleton, 1996); (7) fluctuations in the marine oxygen-isotope record (Miller et al., 1987, Kennett and Stott, 1991, Pak and Miller, 1992, Thomas and Shackleton, 1996); and (8) a large increase of kaolinite in the clay mineral suite (Robert and Chamley, 1991, Robert and Kennett, 1992, Robert and Kennett, 1994, Gibson et al., 1993, Knox, 1998).

Oxygen isotopes provide evidence for an increase in both oceanic bottom- and surface-water temperatures in the higher latitudes during the latest Paleocene (Thomas and Shackleton, 1996). A higher latitude increase in both air temperature and amount of precipitation also is indicated by the occurrence of relatively high proportions of kaolinite in uppermost Paleocene and lowermost Eocene deposits in the South Atlantic off Antarctica (Robert and Chamley, 1991), the northeastern North Atlantic (Robert and Chamley, 1991, Knox, 1996, Knox, 1998), the northwestern North Atlantic (Gibson et al., 1993), and New Zealand (Kaiho et al., 1996).Robert and Kennett, 1992, Robert and Kennett, 1994 suggest that the late Paleocene kaolinite increase was a result of increased leaching, and they propose that globally warm conditions at this time reduced the pole-to-equator temperature gradient and warmed the high latitudes disproportionately. This warming was accompanied by enhanced evaporation at low-to-middle latitudes, which would favor increased formation of palygorskite and/or sepiolite, and by increased precipitation at higher latitudes, which would favor the formation of kaolinite.

We examined clay mineral suites from Paleocene/Eocene (P/E) boundary strata in northern Atlantic Coastal Plain sites throughout the Salisbury embayment and from deeper water strata in Deep Sea Drilling Program (DSDP) Site 605 on the upper continental rise off New Jersey (Fig. 1). We also attempted to document the southern latitudinal extent of intensified latest Paleocene weathering in the eastern US. P/E boundary strata have not been recognized in the southern Atlantic Coastal Plain. Upper Paleocene and lower Eocene strata do occur in Alabama and eastern Mississippi, however, (Gibson et al., 1982, Ingram, 1991, Gibson and Bybell, 1995) and clay mineral suites were examined from several localities.

Robert and Kennett (1994) estimated the extended period of increased kaolinite formation in the latest Paleocene to have lasted about 150 k.y. Bralower et al. (1998) considered the late Paleocene thermal maximum (LPTM), which includes the period of high kaolinite formation, to have a maximum duration of 200 k.y. Sloan and Thomas (1998) suggest an even shorter time period of 10,000–100,000 years for LPTM events. By all accounts, this distinctive kaolinite pulse represents a short time period that offers considerable potential as a fine-scale correlation tool for comparing complete and incomplete sections across the P/E boundary.