POTENTIAL AFFECTS OF VERTEBRATE BURROWS ON FLUID FLOW: AN EXAMPLE FROM THE MORRISON FORMATION, SOUTHEAST UTAH

Large-diameter structures (LS) in the Salt Wash Member (SWM) of the Upper Jurassic Morrison Formation near Ticaboo, Utah, are interpreted as vertebrate burrows. The LS can be traced in outcrop for over 2 km. The LS have four morphotypes, but only one is vertically oriented to potentially affect fluid flow between layers. This LS1 is the most common and composed of a vertical to subvertical helical shaft > 1 m deep that leads to a shorter horizontal tunnel. The complex architecture of LS1 resembles fossil and extant synapsid burrows. Ridge and knob textures on the surface are interpreted as marks produced during excavation and fossilization. All LS are preserved by illuviated carbonate, a lithology separate from the surrounding siltstone and under- overlying sandstone. The surfaces of the LS show cracks from expansion. The SWM consists of interbedded sandstone, siltstone, and mudstone interpreted as fluvial deposits. The depositional environment of the LS-bearing units is interpreted as a proximal to distal floodplain paleosol. The LS-bearing siltstones are near the top of the SWM. Sandstones with pebble conglomerate lags interpreted as bars and crevasse-splay events are interbedded with the LS-bearing paleosols and unburrowed siltstones. The paleosols contain many slickensides and pseudo-anticlines. The difference in lithologies between LS fill and matrix will have an effect on the movement of groundwater and hydrocarbons. LS may act as macrochannels not through hollow tubes, but through the annulus between the carbonate and siltstone or sandstone – the ring of space between the different rock types. Their effect will be dependent on their density and distribution. Density is 9 LS/10 m² on average, though their distribution is not uniform across the outcrop. LS macrochannels can increase permeability, preventing the siltstone matrix from acting as a barrier to fluid flow and improving connectivity between underlying and overlying sandstones. Increased awareness of potential vertebrate marcropores is important. Applying spatial ichnologic and sedimentologic data from this and previous studies of the SWM near the study area can greatly improve our understanding of how terrestrial bioturbation affects subsurface fluid flow.