Site 1212 is located in middle bathyal (2681 m) water depth on the southern flank of the Southern High of Shatsky Rise. The site is at the location of DSDP Site 47 (Fischer, Heezen, et al., 1971). Hole 47.2 was cored with RCB penetrating 129.2 m of predominantly soft sediments, which frequently were so fluid that cores could not be opened. The hole terminated at a hard layer interpreted as chert. The recovered sediments, mainly nannofossil ooze and chalk are Pleistocene to late early Maastrichtian in age. Site 1212 also lies about 1.3 km to the northwest of DSDP Site 577.
According to the drilling record from Hole 47.2 (Fischer, Heezen, et al., 1971), the uppermost seismic reflector correlates with a major unconformity between the upper Miocene and the lower Oligocene. The next highest reflector corresponds to the uppermost chert horizon in the Maastrichtian. The interval between the two reflectors contains a Paleogene to uppermost Cretaceous section with minor unconformities. The lower Eocene to the K/T boundary section in Hole 47.2 was apparently complete and included the PaleoceneEocene transition. The K/T boundary was described as heavily disturbed by coring but the described planktonic foraminiferal faunal succession was complete (Douglas, 1971). Abundant fragments of Inoceramus shells were recorded from the highly disturbed nannofossil ooze in the lowermost core taken (Core 47.2-14; 128.0 to 129.2 mbsf). Stable isotope analyses on single planktonic species have been performed on samples from this site and from other Shatsky Rise sites including Sites 305 and 306, and the first paleotemperature curves for the Northern Pacific were constructed for the entire Pleistocene to Cretaceous interval (Douglas and Savin, 1971, 1975). LPTM excursion isotope values were identified on single specimen and multispecimen analyses of foraminifers from Hole 47.2 by Stott (1992). This interval was described as highly disturbed by drilling.
Site 1212 is the third shallowest site in the Shatsky Rise depth transect. The site is approximately 300 m deeper than Site 1209 (2387 m) and 230 m shallower than Site 1211, the deepest site drilled on the Southern High; Central High Site 1208 at 3346 m is ~770 m deeper then Site 1212. The objective at Site 1212 was to recover a complete and undisturbed record of the Hole 47.2 sequence with APC double coring. As part of the Shatsky Rise depth transect, drilling at Site 1212 addresses a number of leg-related objectives.
Holes 1212A and 1212B were cored with the APC (Table T1). Hole 1212A terminated at 101.6 mbsf at the uppermost chert layer in the upper Maastrichtian. In Hole 1212B, 11 chert layers were penetrated with XCB center bit drilling and a total of 207.6 mbsf was cored, reaching the upper Albian.Summary of Results
Coring at Site 1212 recovered three lithologic units that have been separated based on sediment composition and color variation (Fig. F34). Lithologic Unit I ranges from Holocene to middle Miocene (0 to ~15.1 Ma, 063.0 mbsf) and consists of nannofossil ooze with clay, clayey nannofossil ooze with foraminifers, and clayey nannofossil ooze, light gray to light olive-gray in color. This unit has a higher clay content than underlying units. Two subunits are distinguished. Subunit IA (Holocene to upper Pliocene, 0 to ~3 Ma, 035.9 mbsf) is characterized by interbedded light gray nannofossil ooze with clay and light olive-gray clayey nannofossil ooze with foraminifers expressed as decimeter-scale light/dark alternations. The clay and foraminiferal content varies from 5% to 35% and from 0% to 32%, respectively. Minor components include siliceous microfossils (diatoms, radiolarians, silicoflagellates, and sponge spicules) and rare ash layers. Subunit IB (upper Pliocene to middle Miocene; ~3 to ~15.1 Ma; 35.963.0 mbsf) consists of light gray nannofossil ooze with clay and light olive-gray clayey nannofossil ooze and contains subtler decimeter-scale light/dark alternations. Siliceous microfossils are present but considerably rarer than in Subunit IA. Ash layers are absent.
An unconformity from middle Miocene to lower middle Eocene separates lithologic Unit I from Unit II (see "Biostratigraphy" in "Special Syntheses"). Lithologic Unit II ranges from lower middle Eocene to lower Paleocene (~43.6 to 65 Ma; 63.0102.2 mbsf) and consists of alternating pale yellowish brown nannofossil ooze with clay and pale orange to grayish orange nannofossil ooze (Subunit IIA; lower middle Eocene to Paleocene/Eocene boundary; 43.6 to 55 Ma; 63.079.9 mbsf), and very pale yellowish brown nannofossil ooze with clay interbedded with very pale orange nannofossil ooze (Subunit IIB, Paleocene/Eocene boundary to K/T boundary; 55 to 65 Ma; 79.9102.2 mbsf). A number of minor diastems occur in this interval as indicated by thin, darker horizons. The unit has a generally higher carbonate content than Unit I.
Lithologic Unit III ranges from upper Maastrichtian to upper Albian (65 to ~100 Ma, 184.108.40.206 mbsf) and consists of soft, white nannofossil ooze and nannofossil ooze with foraminifers interbedded with chert. The unit has extremely high carbonate contents and is frequently highly disturbed by drilling because of its fluid nature. Eleven chert layers were penetrated. Two significant unconformities are found toward the base of this unit, separating the upper Campanian from the Coniacian and the Coniacian from the upper Albian.
The Site 1212 stratigraphic section shows broad similarity to the sections recovered at previously drilled Sites 12091211 suggesting, in general, common sedimentation histories. In particular, the critical boundaries, the Paleocene/Eocene and Cretaceous/Paleocene, show a similar sequence of lithologies. In addition, sedimentation rates throughout the Site 1212 section are generally comparable with the rates estimated at Sites 1209 and 1210, whereas they are higher than at Site 1211 in certain intervals (e.g., PaleoceneCampanian; Fig. F30). The major difference is the ~30 m.y. unconformity between lithologic Units I and II, a gap that is longer in duration than the major NeogenePaleogene unconformity at other Southern High sites. Additional unconformities and condensed intervals in the Site 1212 section may be located in shore-based biostratigraphic investigations.Highlights
The stratigraphic record at Site 1212 reveals differences from the records at Sites 1209, 1210, and 1211 that are not related to water depth. For example, the major MioceneEocene unconformity spans the lower Oligocene through mid-middle Eocene, an interval largely preserved at the other sites, including Site 1211, which is situated 230 m deeper than Site 1212. This suggests that the missing section at Site 1212 is not a result of dissolution. The absence of upper Eocene to Oligocene sediments at Site 1212 is more likely a result of local erosion or mass wasting. The site lies near the top of a canyon and in an area of the Southern High where the stratigraphy is complex (Sliter and Brown, 1993). Thus, it is possible that mass wasting has removed part of the section in the vicinity of Site 1212.
The major highlight of coring at Site 1212 is similar to the highlights at Sites 1209, 1210, and 1211, namely the recovery of all of the critical intervals, most in both holes. One exception to this is the Eocene/Oligocene boundary, which lies within the Miocene to Eocene unconformity. Even though coring of Cretaceous sediments was limited, the results from Site 1212 are meaningful with regard to the geologic history of Shatsky Rise.Recovery of Critical Events
Coring at Site 1212 recovered the LPTM, the Cretaceous/Paleocene boundary, and the MME. The LPTM at Site 1212 is lithologically similar to Sites 1209 and 1210, corresponding to a 9-cm interval of yellowish brown clayey nannofossil ooze. This lithology has a sharp contact with underlying pale orange nannofossil ooze, and a gradational contact with overlying pale orange nannofossil ooze. Preliminary biostratigraphic investigation suggests that the sequence is similarly expanded to the Sites 1209 and 1210 LPTM sections and that similar changes in assemblages and microfossil preservation occur within the event. For example, the abundance of Fasciculithus decreases sharply near the onset of the event, and excursion foraminifers (e.g., Kelly et al., 1996) have been observed. The base of the LPTM event, however, appears to correspond to an unconformity. Foraminifers in Sample 198-1212B-9H-5, 7778 cm, include Globanomalina pseudomenardii, the LO of which defines the top of Zone P4 at 55.9 Ma (age), ~0.4 m.y. before the onset of the event. The significance of this unconformity in terms of stratigraphy and paleoceanographic interpretation of the LPTM is currently not understood.
The K/T boundary interval is also lighter in color than at the other sites and the sediments near the boundary appear to be somewhat less indurated. Ooze immediately underlying the boundary correlates to uppermost Maastrichtian nannofossil Zone CC26 based on the presence of Micula prinsii. The paleontological boundary has been bioturbated, and careful sampling of burrows of lowermost Danian sediment that extend 10 cm into the uppermost Maastrichtian yields ~100-sized, amber to olive-green spherules as well as tiny planktonic foraminifers possibly representing basal Paleocene Zone P0. Planktonic foraminifers in the lowermost Danian correspond to Zone P. The identical faunal and floral changes are observed in the boundary interval as at the other sites, and the early Danian interval, in particular, appears rather expanded and remarkably undisturbed.
Because Site 1212 is located close to Site 577 where the K/T boundary was triple cored during Leg 86 (i.e., Wright et al., 1985), comparison is necessary. Lithologically, the K/T boundary recovered in Holes 577, 577A, and 577B are similar although not identical to the Site 1212 record. The main difference between the sections is the degree of bioturbation and thickness of the bioturbated layer, and the thickness and color of the lowermost Paleocene foraminiferal ooze layer. Lighter color is associated with an increase in thickness of the basal layer from 10 cm in Hole 577A to more than 15 cm in Hole 577. Smear slide analysis for Site 577 showed that planktonic foraminifers are abundant (up to 35%) from basal Paleocene to ~6070 cm above the K/T, whereas above and below this interval they rarely exceed 1%. At Site 1212, abundant foraminifers are found in smear slides only in the lowest 1020 cm of the basal Paleocene.
A detailed study of planktonic foraminiferal biostratigraphy and assemblages across the K/T boundary at Site 577 was conducted by Gerstel et al. (1986). These authors illustrated the same evolutionary trends and succession of events as found at Site 1212 and other Leg 198 sites. Gerstel et al. (1986) argued that the presence of Parvularugoglobigerina eugubina below the K/T boundary was evolutionary. However, we have noticed that (1) the distribution of P. eugubina below the boundary is confined to the burrows associated with tektite-like spherules and (2) it is preceded in the deepest burrows by the tiny Guembelitria assemblage reminiscent of the P0 zonal fauna. Moreover, the Site 1212 record shows that the tektite-like spherules are concentrated in the first few centimeters (23 cm) above the K/T boundary, and their record above is related to the intense burrowing. Nannofossil assemblages across the boundary were described in detail by Monechi (1985), who documented that the basal Paleocene yellow-brown ooze is characterized by an abrupt increase of Thoracosphera and the FO of Neobiscutum romeinii, followed by the FOs of Markalius astoporus and Cyclagelosphaera reinhardtii, similar to the sequence of events found at Site 1212 and other Leg 198 sites.Cretaceous History of the Southern High
The MaastrichtianCampanian section at Site 1212 is underlain by a thin layer of clay and glauconite-rich ooze of Coniacian age (planktonic foraminiferal Zone KS23, top of Section 198-1212B-24H-6), which in turn is underlain by ooze of late Albian age (Zones KS 1617; base of Section 198-1212B-24H-6 to Section 27H-CC). This interval has been recovered at a number of other sites on Shatsky Rise, including Sites 1207 and 1213, and DSDP Sites 305 and 306 (Luterbacher, 1975). Coniacian to upper Cenomanian sediment, however, is extremely rare on Shatsky Rise as noted by Sliter (1992), who also reevaluated the planktonic foraminiferal biostratigraphy of Site 305 and found an unconformity in this interval. Sliter (1992) proposed that the Coniacian to Cenomanian interval was widely unconformable as a result of deep-sea erosion or dissolution. Coring results from Site 1212 and other Leg 198 sites confirm this conclusion. The fact that the one site where the Cenomanian to Coniacian interval is partially recovered, Site 1207 on the Northern High, lies at greater depth (3101 m) than Sites 1212 and 305 (2903 m) where it is unconformable, suggests that erosion is the likely cause of the regional unconformity. In addition, seismic line TN037-17A, which crosses Site 1209, 1210, and 1211, shows a prominent unconformity that likely correlates to the Cenomanian to Coniacian interval, where mid-Cretaceous dipping horizons are cut by horizontal uppermost Cretaceous horizons (Fig. F29). Thus, the stratigraphy of Site 1212 has important implications for the Cretaceous history of Shatsky Rise.
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