FLORIDA GEOLOGICAL SURVEY


packstone with varying amounts of quartz sand
and clay (Scott, 1988). Minor phosphate (less
than 3 to 5 percent), dolomite and chert
(siliceous limestone, silicified corals; see also
Upchurch et al., 1982) are also observed. Fossil
molds of foraminifera, mollusks, gastropods and
algae are all common within the Tampa Member
(Scott, 1988). Pinkish gray to light olive gray
dolostones also occur with a similar accessory
mineral assemblage and fossil assemblage as the
limestones. Thin sand and clay beds can be
found sporadically within the unit (Scott, 1988).
Porosity of the Tampa Member is generally
intergranular and moldic, with measured total
porosity values (17 samples) ranging from 10.4
percent to 49.6 percent, averaging 32.3 percent
(median value =33.6 percent).

 The subcrop limit of the Tampa Member
extends from Pasco County to the northernmost
part of Charlotte County and eastward into the
western half of Polk, DeSoto and Hardee
Counties (Plate 49 and 50). The top of the
Tampa Member ranges from more than 100 ft
(30.5 m) MSL in Pasco County to deeper than -
350 ft (-107 m) MSL in Sarasota County (Plate
49) and exhibits variable thickness. The
maximum observed thickness of the Tampa
Member is 292 ft (89.0 m) (Plate 17; W-14882
[TR 6-1]); however, some would propose that
the lower Tampa Member in this well is more
characteristic of the undifferentiated Arcadia
Formation.

 In the northern third of its extent, the Tampa
Member overlies the Suwannee Limestone and
the contact appears to be locally conformable.
In Pinellas and northwest Hillsborough
Counties, for example, samples have been
informally referred to as "SuwTampaHaw" due
to the subtle transition between the units. In the
central and southern regions, this unit overlies
the Arcadia Formation (undifferentiated), or the
Nocatee Member of the Arcadia Formation. In
many wells, the transition between the Tampa
Member and the underlying Arcadia Formation
is gradational, with phosphorite content
increasing with depth. The Tampa Member is
conformably overlain by the Arcadia Formation
(undifferentiated) in many areas (e.g., Plate 14);
however numerous exceptions exist. In parts of


Pasco and northern Hillsborough Counties, the
unit is unconformably overlain by UDSC or
undifferentiated clay-rich Hawthorn Group
sediments. East of this area, the Tampa Member
is unconformably overlain by the Peace River
Formation. Toward the east and south, the
Tampa Member facies grades laterally into the
Arcadia Formation. In Sarasota County, the unit
appears to grade laterally into the Nocatee
Member as it becomes increasingly more sandy.
Scott (1988) also reports this lateral facies
change in northern Hardee County.

 The Tampa Member generally exhibits
variable gamma-ray activity (Figure 10; Arthur
et al., 2001a) that limits the value of this log to
discern unit boundaries. For example, when
underlain by the Arcadia Formation
(undifferentiated), the Nocatee Member or the
Suwannee Limestone, it is difficult to
distinguish these units from the Tampa Member
based on gamma-ray activity. On the other
hand, where the Tampa Member is overlain by
the Arcadia Formation, the two units are usually
readily distinguishable due to higher gamma-ray
activity in the undifferentiated Arcadia
Formation.

 Along the updip erosional pinchout of the
Tampa Member, where it forms an irregular
subcrop contact with the Suwannee Limestone
(Plate 49), the top of the FAS generally
coincides with the uppermost carbonate unit
occurrence (Figure 8). In the west-central part
of the study area, the Tampa Member is the
uppermost lithostratigraphic unit within the FAS
(Figure 8); however, based on lithologic and
hydrologic data from wells in south-central
Hillsborough County, the Tampa Member is
locally hydraulically separated from the
Suwannee Limestone and is therefore considered
part of the IAS/ICU (Figure 8). This latter
hydrogeologic setting occurs locally in northern
Pinellas County as well.

 The depositional environment of the Tampa
Member was that of a quiet water lagoon, much
like present day Florida Bay (King, 1979). An
influx of siliciclastics nearly devoid of
phosphorite distinguishes Tampa Member
deposition from older and younger Hawthorn
Group units in the stratigraphic section.