Abstract: Stephanian A and B Alluvial-Fan and Lacustrine Sedimentary Rocks from La Magdalena, Matallana, and Sabero Coalfields, Cantabrian Mountains, Northern Spain
Alan P. Heward, John A. Knight, Robert H. Wagner
Detailed stratigraphic mapping and vertical-sequence and paleocurrent analyses allow distinction of six sedimentary associations in terms of depositional environments on, or related to, alluvial fans fringing a fault-bounded sedimentary source region. These environments are: (1) topographic-valley and fanhead-canyon fills: diagenetically reddened, scree, and conglomeratic thinning and fining-upward sequences; (2) alluvial-fan channels: conglomerate or coarse-sandstone filled; (3) mid-fan conglomeratic or sandstones lobes: laterally extensive, thickly bedded, and typically comprising a downfan sequence of unstratified conglomerates-stratified conglomerates-pebbly sandstones-cross-bedded sandstones; (4) interlobe and interchannel: siltstones with abundant floras, thin coals and upright tree trunks; (5) distal fan: 10 cm to 1-m thick sheet sandstones which preserve numerous upright tree trunks, separated by siltstones with abundant floras and coals; the sheet sandstones are arranged in sequences of progressively changing grain size and bed thickness; (6) lacustrine: coals, limestones, and/or fine-grained, low-energy coarsening-upward sequences.
Proximal-fan sediments are not preserved. Mid-fan depositional processes consisted of channel-fill, debris flow, and turbulent-water flow, but sheet flooding dominated active distal areas. Abandoned fan surfaces were colonized by vegetation and a ?tropical seasonal climate? resulted in the diagenetic reddening of a small part of the sediments. Worked coals (up to 20 m thick) are in the distal fan and lacustrine environments.
The fans infilled fault-controlled "California-like" basins, deformed and now preserved along major structural lines. This type of predominantly continental coalfield is not unique to northern Spain and an understanding of the depositional setting has potential in coal exploration.
AAPG Search and Discovery Article #90968©1977 AAPG-SEPM Annual Convention and Exhibition, Washington, DC