Biostratigraphy and carbon isotope stratigraphy of uppermost Cretaceous‐lower Cenozoic Muzzle Group in middle Clarence valley, New Zealand

Abstract

Muzzle Group strata exposed along southeast-flowing tributaries of the Clarence River valley, Marlborough, record hemipelagic-pelagic sedimentation across a high latitude (c. 55°S), terrigenous sediment-starved, continental margin from latest Cretaceous to middle Eocene times. Studies of dinoflagellates, foraminifera, calcareous nannofossils, and radiolarians have been integrated with bulk carbonate δ 13 C profiles to establish the chronostratigraphy for two stratigraphic sections along Bluff and Muzzle Streams, middle Clarence valley. The two sections comprise similar successions. Uppermost Cretaceous (upper Haumurian) micritic limestone of Mead Hill Formation is overlain unconformably by Teredo Limestone, a c. 0.25 m thick bed of highly glauconitic, calcareous sandstone. This unit, the basal member of Amuri Limestone, is overlain conformably by c. 15 m thick Lower Limestone, micritic limestone that is glauconitic at base and progressively more marl-rich in its upper part. Lower Limestone grades up into Lower Marl, a poorly exposed, 40-70 m thick unit of alternating marl and micritic limestone beds. Biostratigraphy indicates that the base of Amuri Limestone is younger at Bluff Stream (earliest Eocene, early Waipawan) than at Muzzle Stream (late Paleocene, late Teurian). In the condensed (12 m) upper Paleocene-lower Eocene Amuri Limestone sequence at Muzzle Stream, a trend in δ 13 C from high ( 2.4‰) to low ( 1‰) values is consistent with global records across three major climate or carbon cycle perturbations: the late Paleocene carbon isotope maximum (PCIM, 59-56 Ma), the initial Eocene thermal maximum (IETM, 55.5 Ma), and the early Eocene climatic optimum (EECO,. Probably only the upper PCIM is preserved in the 4 m thick siliceous limestone interval overlying Teredo Limestone. The IETM is well-defined by a 1‰ negative δ 13 C excursion at the base of a 0.8 m thick marl-rich unit (Dee Marl), 5 m above the base of Lower Limestone at Muzzle Stream, and the abrupt appearances of Eocene-restricted species or distinctly warm-water elements within dinoflagellate, foraminiferal, calcareous nannofossil, and radiolarian assemblages. The lithological expression of the IETM as a recessive marly unit has now been identified in three Clarence valley sections (Muzzle, Dee, and Mead Streams), representing a 20 km continental margin transect. Sedimentation rate trends across this margin indicate that the local effects of extreme global warming were increased supply of terrigenous mud, probably due to enhanced precipitation, weathering and erosion, and a decrease in pelagic sedimentation, reflecting a decrease in oceanic productivity. Bluff section lacks an IETM record but contains an expanded (20 m) early Eocene succession that records the onset of the EECO as a progressive

Large-Amplitude Variations in Carbon Cycling and Terrestrial Weathering during the Latest Paleocene and Earliest Eocene: The Record at Mead Stream, New Zealand

“…However, previous work suggests the origin of this interval differs: it generally relates to carbonate dissolution and lower sedimentation rates in the deep ocean (e.g., Lourens et al 2005;Zachos et al 2005;LeonRodriguez and Dickens 2010;Stap et al 2010;Zachos et al 2010) and carbonate dilution and higher sedimentation rates along continental margins (Schmitz et al 2001;Hollis et al 2005aHollis et al , 2005b; Giusberti et al 2007;Nicolo et al 2007;John et al 2008) and at high latitudes .…”

“…We have continued such logging through Lower Marl until ∼285 m above the K/T boundary. Deformation (e.g., boudinage, shear, faults, and isoclinal folds) complicates logging above this level, although good stratigraphic sections do exist further upstream, as well as along other streams in Clarence River Valley (Reay 1993;Hollis et al 2005b). …”

“…Across Clarence River Valley sequences, thicknesses of all lithological units generally increase with paleodepth (Reay 1993), being thickest near the depocenter as represented by the Mead Stream section. This is exemplified by Dee Marl, the unit that marks the PETM, which thins toward the paleo-coast, being 2.4 m at Mead Stream (Hollis et al 2005a), 1.0 m at Dee Stream (Hancock et al 2003), and 0.8 m at Muzzle Stream (Hollis et al 2005b). Thus, a progressive increase in paleodepth over the 2-3 m.yr.…”

“…However, the thickness of Dee Marl relative to the thickness of surrounding Lower Limestone has an opposite relationship. The 1.0-and 0.8-m-thick intervals spanning the PETM at Dee Stream and Muzzle Stream represent much larger percentages of the total thickness of Lower Limestone in these sections (Hancock et al 2003;Hollis et al 2005b). …”

“…Second, the coupling between temperature and carbon cycling is skewed in the time domain, such that the Figure 7. Radiolarian, foraminiferal, and nannofossil biostratigraphic age datum comparison between site 1262 (Zachos et al 2004;Agnini et al 2007) and Mead Stream (Hollis et al 2005b). Ages and heights/depths are plotted relative to the onset of the Paleocene-Eocene thermal maximum (PETM).…”