Closely spaced smear slides were examined from selected intervals of Hole 1223A sediments in order to determine the distribution and general characteristics of microfossils in these sediments in anticipation of our routine micropaleontological preparation procedure. The results of these observations are shown in Table T1, which includes a brief description of the microfossils we observed, our estimate of the frequency of Radiolaria in each sample, and a qualitative observation of the concentration of silt-sized clastic grains. High to very high concentrations of clastic grains are interpreted to indicate the basal intervals of turbidite beds. Low to moderate concentrations of clastic grains indicate the fining-upward portions of turbidites or purely pelagic intervals interbedded within stacked turbidites.
With the exception of single specimens of small foraminifers at 0.6, 5.3, and 5.4 mbsf, Radiolaria are the only microfossils observed in these smear slides. Stephens, Kasahara, Acton, et al. (2003) reported occurrences of sponge spicules in lithologic Unit 1, but we found no evidence of these fossils in our slides. We observed abundant occurrences of fine tabulate crystals at 1.9 and 2.0 mbsf, which coincide approximately with the depths given by Stephens, Kasahara, Acton, et al. (2003) for sponge spicules, but the crystals we observed lack the characteristic morphology of sponge spicules. Similar occurrences were observed in the interval 7.33–7.81 and 38.69–38.83 mbsf. These crystals may be dispersed opal-CT blades or zeolitic minerals.
In general, the distribution of Radiolaria in Hole 1223A sediments can be divided into four intervals (A–D). Interval A comprises the depth range 0–3 mbsf and occupies the upper half of lithologic Unit 1 (Sections 200-1223A-IH-1 through 2). Of the 32 smear slides, 25 are barren of microfossils. Seven samples contain small fragments of skeletal lattice in frequencies of 13–26 specimens/cm2. Many of these specimens are encrusted with pyrite or manganese, which is common for red clays. The high frequency of lattice breakage is consistent with turbidite transport. These occurrences were found in pelagic clays from 0.18 to 0.6 mbsf (Subinterval A1) and in the turbidite beds from 2.07 to 2.4 mbsf (Subinterval A2). Opal-CT pseudomorphs or casts of Radiolaria are locally abundant throughout Interval A.
Both preservation state and frequencies of occurrence improve markedly in Interval B, which occupies the depth range 3.02–7.1 mbsf and includes the lower half of lithologic Unit 1 and essentially all of lithologic Unit 2. Radiolaria are present as larger fragments or whole specimens, but occurrences are sporadic. In Interval B, 16 of the 41 smear slides are barren of microfossils. The distribution of Radiolaria in Interval B has been divided into four subintervals (B1–B4), which may be associated with either clastic-rich or clay-rich sediments. Subinterval B1 occupies the depth range 3.02–3.36 mbsf and includes both clay and silt beds. The sample at 3.16 mbsf contains the second highest frequency of Radiolaria (460 specimens/cm2) and can be described as Radiolaria ooze. Subinterval B2 occupies the depth range 3.76–3.96 mbsf and has Radiolaria frequencies of 40–171 specimens/cm2. The clay-rich sediments underlying Subintervals B1 and B2 are barren of microfossils. Subintervals B3 and B4 occur in basal lithologic Unit 1 and upper to middle lithologic Unit 2. These two subintervals are separated by a barren interval 40 cm thick. The top of Subinterval B3 spans the top of lithologic Unit 2, from 5.1 to 5.4 mbsf. Only modest frequencies of Radiolaria (13–118 specimens/cm2) occur in this subinterval. Subinterval B4 occupies the depth range 5.8–7.1 mbsf. Radiolaria frequencies are quite variable but range up to the highest frequency observed in Hole 1223A sediments (945 specimens/cm2 at 6.7 mbsf). This latter sample is remarkable for the abundance and excellence of preservation of Radiolaria in association with very high concentrations of volcaniclastic grains.
Microfossils were not observed in Interval C, which includes lithologic Units 3–11 and occupies the depth range 7.1–36.99 mbsf. We did not examine the black sands and vitric tuffs of lithologic Units 5, 8, and 11.
Interval D occupies the depth range 36.99–38.7 mbsf and includes the volcaniclastic silty claystones and clayey siltstones (lithologic Units 12–14) immediately underlying the palagonitized vitric tuff of lithologic Unit 11. Very poorly preserved Radiolaria in frequencies of 13–131 specimens/cm2 are consistently present in these sediments. Opal-CT recrystallization has occurred through this interval. Identifiable specimens of Radiolaria are sparsely present in the upper portion of this interval, but preservation state degrades with depth.