Abstract:
Eocene to Oligocene marine carbonate shelf deposits in a continuous core in Baldwin County, Alabama, were evaluated within a sequence stratigraphic framework to determine characteristic stable isotopic responses to inferred fluctuations in sea level. Except for the Eocene/Oligocene boundary interval, the carbonates were deposited landward of their respective depositional shelf edges. Sequence boundaries, transgressive surfaces and downlap surfaces associated with condensed sections (zones of reduced sedimentation) were delineated by lithologic variations in slabbed core, in thin section, and with gamma logs. Because the stable isotopes are recording different aspects of sea level history, it may be necessary to evaluate isotopic signatures independently. In the sequence stratigraphic framework, the delta (super l3) C signature mimics relative sea level changes (paleobathymetry). The delta (super 13) C values generally become heavier in shallower water deposits and lighter in deeper water deposits. However, in planktonic oozes, the delta (super l3) C values may shift dramatically to heavier values. The delta (super 13) C values also record a secular shift to heavier isotopic values from the early Eocene to the Oligocene of about 3.5 per mil. The delta (super 18) O signature records the eustatic sea level history, and preserves only a minor relative sea level signal. The delta (super 18) O values become heavier as eustatic sea level falls and lighter as sea level rises. A secular shift to heavier values from the early Eocene to Oligocene of about 3.0 per mil is apparent in the Bay Minette core. The inflection points of the rise and fall of the delta (super 18) O isotopic curve occur at sequence boundaries and surfaces of maximum starvation. Although minor diagenetic perturbations in the stable isotope signals are found, the depositional signal clearly dominates the isotopic signal. Spar cements generally result in lighter delta (super 18) O and delta (super 13) C values. Dolomitization results in heavier delta (super 18) O and delta (super 13) C values if the dolomitization exceeds about 20% of the rock. This core gives a more complete sea level record than either more landward sections that may have undergone intense subaerial alteration or erosion or from DSDP/ODP records that may be incomplete due to nondeposition. Paleomagnetic and biostratigraphic data obtained from the core enable comparison with correlative DSDP/ODP records. Stable isotopic data in this core reveal more clearly defined trends than have previously been reported from DSDP/ODP records due to a greater sampling density per unit of time.