ORGANIC RICH SEDIMENTS OF THE INDIDURA FORMATION (C/T?), NORTHEASTERN MEXICO

The sedimentary record of the Carbonate platform of northeastern Mexico shows during the Cenomanian/Turonian a facies change from carbonate-rich rocks of the Cupido and Aurora Formations (Barremian and Aptian/Albian respectively) to varying marly limestone interbeds of the Indidura Formation.

Recent high-resolution stratigraphic sampling of the Indidura Formation in the Sierra de Parras shows that the unit exhibits distinct rhythmical interbeds of shales and marly limestones at the macroscopic scale. Lithostratigraphically the succession can be separated into three distinctive clusters where the lower and upper ones are characterized by thick shale beds (up to 2 meters thick), whereas the middle one shows marl-limestone “couplets” with about 10 cm of maximum thickness. Shaly and marly-limestone intervals display fairly regular laminae that are superimposed on the larger-scale beds and are continuous throughout a given bed exposure.

Microscopically, thin section analysis reveals that laminae resemble couplets similar to those found in varves. Microfauna as well as macrofauna are scarce represented by occasional planktonic foraminifera and single Inoceramus valves found at certain intervals.

Preliminary Carbon/Carbonate analysis of the different lithologies shows that Total Organic Carbon (TOC) content is consistently relatively high with values fluctuating from 7.35% to 24.39%, but more commonly higher than 20%. Total Inorganic Carbon (TIC) varies between 43% and 78.3%.

High organic matter accumulation, together with the disappearance of almost all benthic and planktonic forms, as well as exceptionally well preserved laminae, suggest a very low energy environment in which oxygen depletion prevailed throughout the water column. Such conditions are in contrast with those that prevailed at the time of accumulation of the Cupido/Aurora Limestones. Thus, the changes in oceanic environmental conditions that occurred in the northeastern Mexican basin reflect the overall pronounced changes that also occurred elsewhere in the Tethyan domain at that time.