Figure 1.
Terrestrial K/Pg boundary site at Mud Buttes, North Dakota.
(A) Stratigraphic section through K/Pg boundary interval; this section is located ∼150 m southwest of the Mud Buttes burrow locality (adapted from Bercovici et al. 2009). (B) X marks location of Mud Buttes locality in southwestern North Dakota, USA. (C) View of K/Pg boundary in situ at Mud Buttes burrow locality. Note that burrow layer is ∼6 cm above the orange-hued boundary clay at this site. Tic marks at left are in 1 cm increments. (D) Plan view of the undersides of burrows at lignite/silty sandstone interface (specimen KT4/UCM 98213). (E) Close-up of lignitic coal showing poorly-compacted plant debris. (F) Close-up of boundary clay showing abundant spherules ∼40 µm in diameter.
Figure 2.
Planolites isp. burrows from Mud Buttes.
(A) Plan view showing undersides of closely packed horizontal burrows at coal/silty sandstone interface. Note coal still adherent to some burrows (specimen KT3/UCM 98212). (B) Discernible burrows colored and numbered to illustrate minimum burrow density within 8 cm2 (specimen KT4/UCM 98213). Portions of at least 23 burrows comprise roughly 30% of area of white box. Burrows are colored different shades of green to illustrate overlapping relationships. The burrowing activity likely took place within a short period of time, but burrows colored darker green were lower in the soil profile and appear to have been crossed in situ by other burrows (lighter shades of green). Burrows colored lightest green were the topmost burrows. Note that this specimen is the same as that in Figure 1D. (C) Close-up of burrow 19 in B (specimen KT4/UCM 98213).
Figure 3.
Thin sections of bioturbated sediment (A, B) and a section of burrow fill (C, D).
(A) Photomicrograph of thin section of epoxy-impregnated bioturbated layer (specimen KT3-T2b/UCM 98212). Bottom of photo is at the coal/silty sandstone interface, and top is several mm higher in section. The ∼3 mm diameter circular structure labeled “b” in the center of the photograph appears to be a cross section of a burrow. A thin layer of fractured coal is evident below this (“c”). Numerous pelletoid blebs are visible, including one in the burrow cross-section, and several others massed together in the upper part of the photo (two are labeled “p”). (B) QEMSCAN (Quantitative Evaluation of Minerals by SCANning electron microscopy) image of portion of thin section in A showing distribution of clay minerals (brown), quartz (pink), and feldspars (turquoise) in the bioturbated sediment (note that organic matter was not detected at the kV setting used to analyze minerals). Concentrated blebs of clay are conspicuous. Same letter designations apply as in A. (C) Thin section of partial burrow fill from KT3-T3b/UCM 98212. This burrow was sandwiched between the underlying coal and a thin layer of coal on top, indicating burrowing within the Paleocene peat. Red box in lower left shows site of photomicrograph in D. (D) Close-up of area of red-box in C showing a pelletoid clay mass containing tiny (<25 µm) mineral clasts; this clay bleb appears to be a fecal pellet from a deposit-feeding invertebrate. (E) Color key of minerals in QEMSCAN image B and calculated QEMSCAN mass percentages of minerals in bioturbated sediment sample (portion of which shown in B, and in burrow fill shown in C.