ICHNOFOSSILS AS A TOOL FOR CONSTRAINING PEAK SEA LEVELS OF THE LAST INTERGLACIAL HIGHSTAND: AN EXAMPLE FROM EEMIAN STRATA, WEST CAICOS, TURKS AND CAICOS ISLANDS

Carbonate platform features used in numerous published studies to estimate the relative position of Eemian (MIS 5e)/Last Interglacial Highstand sea level include: 1) fossil reef corals, 2) coralline red algal reef caps, 3) exposure horizons, 4) flank margin caves, 5) keystone vugs/fenestral sands, 6) beach to dune facies transitions, 7) ooid shoals, and 8) wave-cut notches. Here we emphasize the importance of ichnofossil distribution in constraining position of the Last Interglacial Highstand using an example in outcrops of Eemian strata on West Caicos Island.

Boat Cove, located along the north-central portion of the island’s west coast, exposes more ooid grainstones compared to the extensive reef deposits farther south. Still, the exposure possesses some corals, red algal caps, and ooid grainstones representing a shoreface to foreshore to eolian dune transition. This allows us to compare the stratigraphic position of key ichnofossils with other commonly used sea level indicators listed above. Here, the highest reef corals with Goniolithon caps occur ~1.80 m above estimated Mean Tide Level (MTL) – a close approximation of Mean Sea Level (MSL). Cross-stratified ooid-rich grainstones that drape the small, isolated mounds of coral-red algal framestone/bindstone also contain a well preserved Ophiomorpha-Planolites-Conichnus trace fossil assemblage. A substantial network of Ophiomorpha (cm-scale dia.) shafts, tunnels, and chambers lie within the first 20 cm of cross-stratified ooid grainstones immediately above the reefal deposits. Although trace density decreases upward within the trough cross-stratified ooid grainstones, Ophiomorpha (both cm-scale traces and mm-scale tunnels of O. puerillis) persist up to 3.3 m above estimated MTL. Modern Ophiomorpha-generating callichirid shrimp rarely burrow at water depths < 1 m in similar Bahamian sands. Gently seaward-dipping beds of grainstone with thin lenses of coarse skeletal hash (foreshore swash zone strata) lie ~1 m higher in the section. Bahamian red algal reef encrusters typically grow within uppermost subtidal to intertidal zones (-0.4 m ± 0.2 m), indicating Eemian sea level at ~2.2 m above current MSL. However, the ichnofossils and foreshore facies place Eemian peak sea level at ~4.3 m above current MSL. Note that relative heights do not include any isostatic adjustments.