VEGETATION INDUCED SEDIMENTARY STRUCTURES AND FOSSILIZED TREES IN TERMINAL SPLAY SANDSTONE BEDS OF THE CRETACEOUS-PALEOCENE RATON FORMATION, COLORADO

Terminal splay bodies preserved in high-accommodation fluvial strata of the Raton Formation (RF) are defined by fine- to medium-grained, tabular, thin (< 1.5 m), and laterally extensive (> 100 m) sandstone beds. Terminal splay sandstones (TSS) feature asymmetrical current and climbing ripples, small scale trough bedding, planar laminations and well preserved plant fossils. TSS are most commonly encased within drab gray, clay rich mudstones interpreted as poorly-drained hydromorphic gleyed paleosols. Basal TSS surfaces are primarily planar, however frequently have concave upward features interpreted as vegetation induced sedimentary structures (VISS).

VISS are primary structures that indicate deposition of terminal splay sheets onto standing trees located on an emergent floodplain. Upright tree fossils and upturned beds in TSS thus represent in situ tree fossils. Coalified root systems and iron concretions nucleated around rootballs are common below upturned beds in underlying gleysols. Rootball concretions are up to 1 m in diameter and coalified roots penetrate up to 1 m into underlying gleysols. Fossilized tree casts in molds in TSS are preserved by coalification or sand infilling. Upturned beds are widest at their base and have widths that range from 20 cm to 2 m, and taper up to the width of the tree fossil, which ranges from 10 cm to 1 m. VISS are readily identifiable due to the planar geometry of basal TSS bounding surfaces, and it is rare that only one upturned bed, rootbed, or tree fossil is found along strike of these units.

The RF is interpreted as having deposited along a humid poorly-drained floodplain with high biologic productivity. Previous studies have identified over 150 fossil plant taxa in the RF, but do not identify their effects on deposition. VISS with sand filled or coalified tree fossil casts and root systems often penetrate multiple TSS. These VISS and tree fossils have the potential to serve as permeability pathways between vertically discrete TSS, increasing connectivity. While this form of connectivity between TSS might seem unimportant individually, VISS are frequent and occur in clusters or at discrete intervals, and may have an important cumulative effect.

This study identifies the effects of vegetation on deposition of ancient terminal splay bodies and their implications on connectivity.