|Explanation: core logs and sedimentary facies|
|gravel (> 60% gravel)|
|gravelly sand (< 3% mud and 25-60% gravel)|
|sand (< 3% mud and < 25% gravel)|
|muddy sand (3-50% mud)|
|mud (> 50% mud)|
|Miocene (limestone and blue-green clay)|
Seven generalized sedimentary-facies types were defined for a unified comparison of core data from the entire study area. All seven color-coded facies for the entire study are shown in the Explanation below. However, not all facies necessarily are present on each transect. Core photographs present individual cores cut into 1-m sections from top (upper left) to bottom (lower right). Discrepancies in core length between the photographs and the diagrams are due to compaction during the coring process. Offshore cores (left) are aligned at core tops. Core locations were chosen to sample thicker Holocene sections and to aid in identifying pre-Holocene stratigraphy. Core elevations were determined from water depth and tide tables. The datum for the barrier-transect cores is the mean lowest low water (MLLW). Core photographs are shown for USGS-95-56 (offshore sand ridge) and AMI-6 (back-barrier).
Offshore vibracore retrieval ranged up to 2.7 m in thickness. Sediments consist dominantly of quartz sand on the surface, occasionally overlying mud and muddy sand, which probably represent back-barrier deposits. The occasionally thick quartz-sand units are probably related to the Tampa Bay ebb-tidal delta and shore-normal bars lying immediately seaward of Anna Maria Island (Gelfenbaum and Brooks, 1997). Seawardmost cores contain a considerable amount of blackened sands.
The island cross section (modified from Yale, 1997) shows sandy nearshore and dune deposits overlapping muddier back-barrier sands to landward. The boundary between "nearshore" sediments overlying "back-barrier" sediments represents a ravinement or maximum flooding surface, and suggest more open marine or lower bay conditions existed in this location prior to the late-Holocene development of northern Anna Maria Island. Beneath northern Anna Maria Island a significant hiatus may be associated with this boundary. Radiocarbon dates from a core located on the landward side of the island, about a kilometer north of the transect yielded a date of 1,040 ± 60 YBP (standard C-14 date) from the base of the nearshore facies, directly and unconformably overlying a brown quartz sand that yielded an AMS C-14 date of 9,730 ± 60 YBP. The brown quartz sand is considered Pleistocene in age, therefore the early Holocene date may reflect some reworking of the surface of this unit during sea-level transgression and ravinement.