What are these basal rip up clasts in North Dakota?
Short answer: They are clasts from adjacent hillside deposited
inmediately after the Chicxulub impact.

The log you posted represents a sudden onshore inundation event due to the impact.
The paper you provided clarifies it [1]:

Tanis site, which documents a turbulently deposited, rapidly emplaced
sediment package directly overlain by the Cretaceous–Paleogene (KPg)
boundary tonstein. The site, situated in the continental Hell Creek
Formation in southwestern North Dakota (Fig. 1), displays
inland-directed flow indicators and holds a mixture of Late Cretaceous
marine and continental biota, implying that its emplacement is related
to sudden onshore inundation surges.

Figure 1: Tanis Site, North Dakota.

Figure 2: Log of Tanis Site, North Dakota.

The unit 1 is described bellow. The contact between conglomerates and the point-bar is non gradational, and the rip-up clasts come from Hell Creek strata above the river (1C).

The base of unit 1 comprises a matrix-supported, massive coarse-sand
conglomerate, with angular pebble- to small boulder-sized rip-up
clasts derived from the underlying Hell Creek strata. As shown in Fig.
2, the massive sand (1a) at the base of unit 1 has a sharp
non gradational basal contact with the underlying point-bar surface and
vertically grades into a thin (∼3-cm) zone of plane-parallel bedding
of interlaminated sand–silt (lower 1b); climbing ripples (mid 1b);
sinuous, wavy lamination (upper 1b); low-angle cross-lamination (lower
1c); fine, discontinuous subparallel lamination (upper 1c); and nearly
structureless fine silt/mud near the top (1d). Flow-direction reverses
180° toward the east—seaward—near the top of unit 1

My guess is they were transported to the sand-bar from the hillside because of the sudden event, before the conglomerate clasts started to fall from the sky.

The whole depositional unit took place shortly, and before the iridium layer.

Because the Tanis deposit contains ejecta throughout and is also
capped by the KPg tonstein, the depositional event took place
during a narrow window of time: after impact but before deposition
of the fine-grained KPg tonstein. Given this constraint, we can
deduce that the Event Deposit was emplaced within a matter of hours
after the Chicxulub impact event.

The subsequent conglomerates are aerial deposits .The event is nicely described at the paper.

We assume a scenario in which ejecta-curtain material, launched at
about a 45° elevation angle and seen as the glassy or altered-glass
spherules at Tanis, arrived before the shocked quartz that was
launched at steep angles in a “warm fireball” produced by release of
CO2 from shocked limestone after departure of the ejecta curtain


 1    Robert A. DePalma, Jan Smit, David A. Burnham, Klaudia Kuiper, Phillip L. Manning, Anton Oleinik, Peter Larson, Florentin J. Maurrasse, Johan Vellekoop, Mark A. Richards, Loren Gurche, Walter Alvarez, (2019):
&quot;A seismically induced onshore surge deposit at the KPg boundary, North Dakota&quot;
Proceedings of the National Academy of Sciences Apr 2019, 201817407; DOI: 10.1073/pnas.1817407116