OPEN-FILE REPORT 92


 The top of the St. Marks Formation ranges from 68 feet (20.7 meters) above MSL (W-
15882, cross-section E-E' on OFMS 99-02) to 8 feet (2.4 meters) above MSL in W-1854 (cross-
section B-B' on OFMS 99-02) within the study area. A maximum thickness of 88 feet (26.8
meters) was penetrated in W-6906 of cross-section E-E' (OFMS 99-02), with a projected
thickness of up to 105 feet in W-15882 (cross-section E-E on OFMS 99-02). The unit pinches
out against the Suwannee Limestone towards the east and south (see cross sections A-A', B-B'
and E-E'; OFMS 99-02). Where present, the St. Marks Formation forms part of the FAS
(Southeastern Geological Society Ad Hoc Committee on Florida Hydrostratigraphic Unit
Definition, 1986).

 Hawthorn Group

 Sediments of the Hawthorn Group (Th) are encountered throughout much of the northern
one-third of the study area where they unconformably overlie the Suwannee Limestone or the St.
Marks Formation (OFMS 99-01). Sediments of the Hawthorn Group are thought to have been
deposited over the Ocala Platform throughout the area, but post-Miocene erosion removed
sediments from the crest of the Ocala Platform exposing the Oligocene carbonates in the
southern portion of the map area (Cooke, 1945; Espenshade and Spencer, 1963; Brooks, 1966;
and Scott, 1981b). Fossils in the Hawthorn Group are sparse but may include vertebrate remains,
corals, and mollusks. Williams et al. (1977) report that the most commonly found fossils are
oysters and coral heads. Within the map area, the Miocene Hawthorn Group (Th) is composed of
the Lower Miocene Torreya Formation (Tht).

Torreya Formation

 The Lower Miocene Torreya Formation of the Hawthorn Group (Tht) is typically a
siliciclastic unit with increasing amounts of carbonate in the lower portion of the unit. The
majority of Torreya Formation outcrops expose the siliciclastic part of the unit which varies from
white to light olive gray, unconsolidated to poorly-indurated slightly clayey sand to light gray to
bluish-gray, poorly-consolidated silty clay often containing a variable but minor component of
carbonate (calcareous or dolomitic). Phosphate grains, while a common but minor lithologic
component of the unit, are often absent (Scott, 1988).
 Several field samples (M-Series) were collected in the southeast quadrant of the Lamont
quadrangle (T 01 S, R 05 E, S 10 and 15), and one sample was collected along the Aucilla River
(T 02 S, R 05 E, S 02) which, at first glance, appeared to be from the Miccosukee Formation
(Tmc). These samples were red to orange clayey sands to sandstones. Upon further examination
under a binocular microscope, however, these samples were determined to have secondary
wavellite cement binding the sands. In these cases, the samples were deemed to be deeply
weathered Torreya Formation siliciclastics (Tht) in which secondary wavellite was precipitated
(Figure 2, OFMS 99-02). Field mapping suggests that the upper surface of the Torreya
Formation is irregular, and the central portions of some hills in the northern part of the mapped
area, which are mapped as Miccosukee Formation, are likely formed by the Torreya Formation.
Where this situation is present, the contact between the two units is difficult to verify due to
downslope migration of Miccosukee Formation sediments as they weather and cover Torreya
Formation sediments.