Tropical Weather in West Texas during the Early Late Cretaceous?*

Transcription

1 Tropical Weather in West Texas during the Early Late Cretaceous?* Brian E. Lock 1 and Fleur Bases 1 Search and Discovery Article #50168 (2009) Posted March 6, 2009 *Adapted from extended abstract prepared for preparation at AAPG Annual Convention, San Antonio, Texas, April 20-23, Geology, University of Louisiana, Lafayette, LA. (belock@cox.net) Abstract The Cenomanian-age Del Rio Formation is unusual within the Cretaceous succession of West Texas, which is otherwise dominated by carbonate sediments. The Del Rio consists of calcareous shales and argillaceous limestones, with scattered quartzose sandstones. The shales contain an abundant but relatively restricted fauna of oysters (Ilymatogyra arietina, primarily, with Exogyra cartledgei in the highest beds), with less common echinoids, amonites, clams, and foraminiferans, including the large distinctive agglutinating species Cribitina texana (was Haplostiche texana). The sandstone beds have distinctive current-oriented prod marks (Figure 1) and other sole marks, possible hummocky cross-stratification in places (Figure 2) and well-developed meter-scale sand waves elsewhere. The sand waves and other prominent beds contain quantities of reworked oysters. The sandstone beds are scarcely burrowed and at least one has well preserved wrinkle-structure (Kinneyia) microbial mat on its upper surface (Figure 3). It is postulated that the sands are products of major storm-surge relaxation currents, which would suggest tropical cyclone conditions in the region during Late Cretaceous. In this regard, the Kinneyia mat is particularly relevant. While cyanobacterial mats are restricted by the need for light for photosynthesis to very shallow water, other bacterial species have been known to form microbial mats in a great diversity of environments: our emphasis on cyanobacterial mats largely reflects the predominance of studies of shallow-water environments. However, the limited literature is consistent in assigning a shallow subtidal to intertidal origin for Kinneyia. Kinneyia is known primarily from Precambrian examples, but rarer Phanerozoic occurrences have been noted. The youngest previously recorded Kinneyia is Jurassic in age, so that the Del Rio example extends the range. Grazing metazoans are blamed for its scarcity after the Cambrian explosion, and young examples are believed to survive only because of unfavorable environmental conditions. Like other Phanerozoic microbial mats, these features are believed to be opportunistic - the bacterial consortia involved are common and widely distributed so that mat development tends to follow quickly on disruption of the prior ecosystem by natural catastrophes such as storms... (Schieber et al., 2007, p. 270) and the mat survives subsequent predation only if adverse environmental conditions persist

2 long enough for the mat to be buried. In this case, the catastrophes were the storm surge currents and the subsequent adverse conditions, possibly the influx of fresh water. The Del Rio Formation environment is suggested to have been a shallow, nearshore shelf experiencing influx of large quantities of suspended sediments in run-off from the nearby land, with fluctuating salinities such that oysters and agglutinating foraminiferans were the most abundant macrofossils. Periodic storms reworked the bottom sediments, forming megaripples and hummocky cross stratification and brought in sand flows. Reference Schieber, J., P.K. Bose, P.G. Eriksson, S. Banerjee, S. Sarkar, W. Altermann, and O. Catuneau, eds., 2007, Atlas of Microbial, Mat Features Preserved within Siliclastic Rock Record: Elsevier, 311 p.

3 Figure 1. Lower surface of one of the tempestite sands, showing oriented sole marks (top right to bottom left of image), and Thalassinoides burrows. Note centimeter scale on card to left of sample. Sandstone bed from Comstock West outcrop.

4 Figure 2. Tempestite beds with hummocky cross stratification, Comstock East outcrop. Note pen for scale. Many of the tempestite beds have concentrations of reworked oysters in the basal part.

5 2008 AAPG Annual Convention & Exhibition; April 20-23, 2008; San Antonio, TX Figure 3. Kinneyia microbial mat on top surface of one of the tempestite sandstone beds. Similar mats have been found at two outcrops east of Comstock and another example was seen near Terlingua, west of the Big Bend National Park, 160 mi (270 km) to the west. Image covers an area 12 inches (30 cm) across.